'Handbook of the Christian Knight' by Desiderius Erasmus (1503)

Gustave Dore, An Angel Leading the Crusaders to Jerusalem, 18--

Finally, my brethren, be strong in the Lord, and in the power of his might. Put on the whole armor of God, that ye may be able to stand against the wiles of the devil. For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places. Wherefore take unto you the whole armor of God, that ye may be able to withstand in the evil day, and having done all, to stand. Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness; And your feet shod with the preparation of the gospel of peace; Above all, taking the shield of faith, wherewith ye shall be able to quench all the fiery darts of the wicked. And take the helmet of salvation, and the sword of the Spirit, which is the word of God: Praying always with all prayer and supplication in the Spirit, and watching thereunto with all perseverance and supplication for all saints.

(Paul's epistle to the Ephesians).

A selection from Erasmus' Handbook of the Christian Knight (Enchiridion militis Christiani, also translated as 'Handbook of the Militant Christian'), 1503.

The story that comes down to us about the origins of this book is that while staying in a castle near a commune in France, Erasmus met a faithless soldier who, at the behest of his complaining wife, asked Erasmus to write him a book on how to live the life of a Christian. The resultant work-- "a handy weapon, an enchiridion, a dagger, that you can always carry with you"-- became one of the most popular pieces of advice literature of the age.

In this life it is necessary that we be on our guard. ... life here below is best described as being a type of continual warfare. Yet ... the great majority of mankind is often deceived, for the world, like some deceitful magician, captivates their minds with seductive blandishments, and as a result most individuals behave as if there had been a cessation of hostilities. They celebrate as if they were assured of victory when, as a matter of fact, genuine peace could never be further away. It is amazing to see in what false security these people live and in what a complacent manner they close their minds to reality. In the meantime the vices, our armored enemies, attack us unceasingly; we are entrapped by their espionage and assulted by their endless deceptions. If you but look around, you will see that regardless of where you go they are observing you. They are prepared to attack us with a thousand stratagems... . Unless we ward them off with the impenetrable shield of faith, they will prove to wield weapons of certain death. Nor is there any slackening in the manner of their attack, as it comes from all sides.
This is that world that st. john describes so well as being constituted entirely of vice. It is a world that is both contrary and hateful to Christ. It must be pointed out that the type of warfare it wages is anything but simple and straightforward. From time to time, especially in adverse circumstances, this raging world shakes the very walls of the mind. ... it incites the mind to betrayal with vain promises. Or again, whenever it finds us unaware, in idle and false security, it unexpectedly and with secret contrivances captures the mind. ... that slimy snake... never ceases to watch and lie in wait... . And yet, as if it were not enough that he threatens us from all directions on the outside, he also penetrates into the inner recesses of our minds.

We practice on the peaceful harp rather than on the weapons of warfare, unaware that this sort of peace is the most terrible of all wars. 

[God] has made it quite plain through His prophet: "There is no rest for the wicked." The condition that He lays down for peace is that we fight in the garrison of the body against all of our vices. If we compromise, if we consort with vice, we will make a foe of Him who alone, as a friend, is able to bless us, but who as an enemy will surely damn us. He will be our enemy for [this] reason: we will be siding with those vices that are diametrically opposed to the divine, for how can light and darkness be in agreement? [...] What could be more shameful, more degrading, than to separate yourself from this princely leader?

Certainly you show the greatest impudence if you dare raise a hostile standard against a King who gave His life for your sake. He Himself has told us clearly that he who does not stand for Him stands against Him, and he who does not gather with Him, scatters.

We do not fight for praise but for Life itself. [...] ...our struggle takes place before the all-seeing eye of God and is witnessed by the entire populace of heaven. The shame of defeat in the presence of such an audience ought at least to help inspire us to bravery. He will praise our effort whose mere approval alone is the equivalent of the greatest happiness. If the tepid mind is not aroused by the prospect of reward, it must be admitted that fear of punishment can awaken even the most indolent. 

Christ referred to the Pharisees as whitened sepulchers. Why? Because they carried their dead souls about within themselves. The bodies of holy people are temples of the Holy Spirit. The bodies of eveil men are sepulchers of dead corpses. No cadaver is so dead as that soul that has been abandoned by God. And certainly no corpse offends the nostrils of men to the extent that the evil odor of the buried soul offends the sensibility of the heavenly court. 

 We must be ever inflamed with the hope and conviction of final victory.

...the outcome of this war is not in the least to be doubted. Victory is not something that depends upon chance; it is entirely in the hands of God and, through Him, also in our hands. Anyone who has failed in this struggle was simply lacking in a will to conquer. 

I think we can truthfully say that nothing is more important in military training than a thorough knowledge of the weapons to be employed and the nature of the enemy to be encountered. I would add to this that the need for preparedness, of having the weapons close at hand, is also of the utmost importance. [...] We can never permit ourselves to be even a finger's length from our weapons. Since our enemy is incessant in his attacks, we must be constantly on the battle ling, constantly in a state of preparedness. As a matter of fact, our enemy, when he appears peaceful, when he feigns flight or a truce, can at that very moment be assumed to be preparing for an attack. He is most dangerous when he appears peaceful, and it is during his violent attacks that we can actually feel most secure. It is for this reason that our primary concern must be to keep the mind armed. 

There is nothing that you can believe with greater certitude than what you read in these writings. The senses themselves cannot offer greater certainty. 

On your right you will find the arms of justice, on your left the armour of truth, the breastplate of justice, and the shield of faith, a shield with which you can ward off the fiery darts of the devil. You will find also the helmet of salvation and the sword of spirit, which is the word of God. Carefully fortified with these weapons, a man can fearlessly utter those courageous words of Paul: "Who shall separate us from the love of Christ? Shall tribulation, or distress, or famine, or peril, or persecution, or the sword?"

We must forge a handy weapon, an enchiridion, a dagger, that you can always carry with you. You must be on guard when you eat or sleep, even when you travel in the course of worldly concerns and perhaps become weary of bearing this righteous armour. Never allow yourself to be totally disarmed, even for a moment, lest your wily foe oppress you. Do not be ashamed to carry this little sword with you. [...] Though it is a small weapon, it will enable you , if you use it skillfully, to withstand the enemy's tumultuous assults quite easily and avoid a deadly wound. Now is the time for us to teach ourselves a kind of "manual of arms." I promise that, if you diligently train yourself in it, our sovereign Lord, Jesus Christ, will transfer you, rejoicing and victorious, from this garrison to the city of Jerusalem, where there is neither tumult nor war at all, but everlasting peace and perfect tranquility. Meanwhile all hope of safety should be placed in your arms and your armour. 

Peace is the highest good.... . Christ alone grants that peace that the world cannot give. There is but one way to attain it; we must wage war with ourselves. We must contend fiercely with our vices. 

...our war is not between man and man, but within ourselves: The hostile battle lines spring forth in opposition to us from our very flesh itself. A friend is distinquished from an enemy by such a fine line that there is great danger of inadvertently defending an enemy as a friend, or attacking a friend thinking him to be an enemy. Our notorious enemy always takes on the appearance of an angel of light. We need always ask, "Are you one of ours or one of our adversaries?" Since you must war with yourself and since the first hope of victory lies in whether you know yourself as much as possible, I shall now put before you a kind of likeness of yourself so that you may plainly know what is within and what is merely skin-deep. 

Man is a very complex creature composed of several contending parts: a soul, which may be likened to a sort of divine will, and a body, comparable to a dumb beast. 

Inasmuch as the body is itself visible, it delights in things visible; inasmuch as it is mortal, it follows things temporal; inasmuch as it is heavy, it sinks downwards. On the contrary, the soul, mindful of its celestial nature, struggles strenuously against the weight of the earthly body to press upward. It distrusts things seen because it knows such things to be transient. It seeks only those things that are true and everlasting. [...] In the past the mind commanded the body without trouble, and the body obeyed freely and willingly. Now, with the natural order of things disturbed [through the fall of man], the passions of the body seek to override the reason... . 
Man, hampered as he is by this perplexing division, may be compared to an unruly state. Such a state is composed of various sorts of men whose dissensions create frequent disturbances and factions. To prevent strife the greatest power must be given to one supreme authority, and this authority must be of such a nature that it commands nothing that is not for the welfare of the state. 

In man, reason discharges the office of king.   [...] Consider the dregs of the lower classes to be those affections or passions that dissent as much as possible from the decrees of reason and that are least humble. These are lust, lechery, envy, and similiar diseases of the mind, which we ought to resist as overseers restrain dirty, vile slaves so as to ensure that they perform the tasks assigned them by the master, or, at least, so as to prevent them from doing harm. The divinely inspired Plato wrote of all these things in his Timaeus. 

Though our king, reason, may at times be oppressed, he cannot be corrupted without protesting. He will be able to recover because of the eternal law that has been divinely engraven upon him. [...] ... the Stoics and the Peripatetics... both agree that we should be guided by reason rather than by passion. ...they contend that the truly wise man must be free of all passions of this sort as diseases of the mind. [...] Socrates, in the Phaedo of Plato, appears to agree with the Stoics when h says that philosophy is nothing more than a meditation upon death, that is, a withdrawal of the mind, as much as possible, from corporal and sensible things, and a dedication to those things that can be perceived only by reason. [...] Socrates' fable of the good and the bad charioteers and the good and bad horses is no old wives tale. 

This then is the only road to happiness: first, know yourself; do not allow yourself to be led by the passions, but submit all things to the judgement of reason. Be sane and let reason be wise, that is, let it gaze upon decent things.

You say that it is difficult to put this advice into practice. Who denies it? Plato has a fitting saying: "Things things which are beautiful are also difficult." Nothing is harder than for man to conquer himself, but there is no greater reward or blessing. 

Most ['of those who bear the name Christian'] are such slaves to their baser appetites that in this spiritual combat they are unable to distinquish between the dictates of reason and the promptings of passion. They actually believe that they are behaving in a reasonable manner so long as they act upon what they feel or see. In fact, they consider that alone to have existence which is perceptible to the senses. Their only critierion for right or wrong is that which appeals to their desires. What they mean by peace is in reality a deplorable state of servitude. ...they follow heedlessly wherever their selfish interests lead. This is that false and unhappy peace that Christ, the Author of peace... has come to do away with. He accomplishes this by stirring up a wholesome war between father and son, husband and wife... . 

I think it is agreed that the authority of the philosophers rests upon the fact that they state what is contained in a different manner in the Scriptures. What the philosophers term "reason" St. Paul calls either "the spirit" or "the inner man" or occasionally the "law of the mind." What they refer to as the "passions" he calls "the flesh," "the body," "the outer man," or "the law of the members." He says, for example, "Walk in the Spirit, and you shall not fulfill the lusts of the flesh. For the flesh lusts against the Spirit, and the Spirit against the flesh... so that you do not the things you would." [ ]And again: "If you live according to the flesh you will die; if, however, you mortify the flesh by the spirit, you will live." Certainly this is a new order of things; to seek peace in war, war in peace, life in death, death in life, freedom in slavery, slavery in freedom. [...] Plato distinquishes two souls in one man. [...] What Paul write to the Corinthians is... pertinent: "The first came from the earth and is terrestrial. The second came from heaven and is celestial." And to make this even more clear he applies this duality not only to Christ and to Adam but to ourselves as well. "As was the earthly man, such also are the earthly; and as is the heavenly man, such also are the heavenly. Therefore, even as we have borne the likeness of the earthly, let us bear also the likeness of the heavenly. This I say, brethren, because flesh and blood can obtain no part of the kingdom of God, neither will corruption have any part in incorruption. 
I think you can see how evident it is that Paul, who elsewhere spoke of the "flesh" and the "outer or corruptible man," here calls him the "earthly Adam." This is certainly the "body of death" about which Paul so frequently speaks. "Unhappy man that I am, who will deliver me from the body of this death?" Pointing out a far different fruit of the flesh and spirit, he writes elsewhere: "For he who sows in the flesh will also reap corruption but he who sows in the Spirit will reap life everlasting." We know that a messanger of Satan came to trouble Paul in the flesh. When the tempter refused to leave him, God gave him this answer: "Paul, my grace is sufficient for thee." For strength is made perfect in weakness. This certainly is a new sort of remedy. Let Paul be proud, he is tempted by pride. That he might be made firm in Christ, he is forced to be infirm. For he carried the treasure of heavenly revelations in a vessel of clay, that the sublimity might reflect the power of God, and not his own power. There are, of course, many other examples in the writings of St. Paul that indicate how we are to overcome temptation. Our first recourse in any kind of temptation is to implore the assistance of Almight God. In fact, those who are well advanced on the road to perfection will actually welcome these temptations, for they guard virtue and especially, form a bulwark against the danger of vanity that often lurks in the midst of the virtues. [...] ... after you have overcome temptation and crucified your flesh with its evil desires, you will find true peace and tranquility, and you will see that the Lord is sweet. God is never perceived in the midst of temptation, but once the temptest subsides, we will bask in the sunshine of spiritual consolation. Examine yourself in all honesty. If you are flesh alone, you will not see God, you will not be saved. Make it your determined effort, then, to become spiritual

Let me sum up how we distinquish these various components of man. The spirit has the capacity of making us divine; the flesh tends to bring out our animal nature; the soul is what really constitutes us as human beings. 

You say that you love your wife simply because she is your spouse. There is really no merit in this. Even the pagans do this, and the love can be based upon physical pleasure alone. But, on the other hand, if you love her because in her you see the image of Christ, because you perceive in her His reverence, modesty, and purity, then you do not lover her in herself but in Christ. You love Christ in her. This is what we mean by spiritual love... . 

It is my plan to propose a number of fundamental rules or norms that will guide us through the labyrinth of this world into the pure light of the spiritual life. If every other science has its own rules then certainly the art of pious living must have some basic regulations. Leading a virtuous life is accomplished by a certain discipline that the Holy Spirit breathes into those who sincerely aim at godliness.