Thou shalt not commit adultery.

In this Commandment too a good work is commanded, which includes much and drives away much vice; it is called purity, or chastity, of which much is written and preached, and it is well known to every one, only that it is not as carefully observed and practiced as other works which are not commanded. So ready are we to do what is not commanded and to leave undone what is commanded. We see that the world is full of shameful works of unchastity, indecent words, tales and ditties, temptation to which is daily increased through gluttony and drunkenness, idleness and frippery. Yet we go our way as if we were Christians; when we have been to church, have said our little prayer, have observed the fasts and feasts, then we think our whole duty is done.

Now, if no other work were commanded but chastity alone, we would all have enough to do with this one; so perilous and raging a vice is unchastity. It rages in all our members: in the thoughts of our hearts, in the seeing of our eyes, in the hearing of our ears, in the words of our mouth, in the works of our hands and feet and all our body. To control all these requires labor and effort; and thus the Commandments of God teach us how great truly good works are, nay, that it is impossible for us of our own strength to conceive a good work, to say nothing of attempting or doing it. St. Augustine says, that among all the conflicts of the Christian the conflict of chastity is the hardest, for the one reason alone, that it continues daily without ceasing, and chastity seldom prevails. This all the saints have wept over and lamented, as St. Paul does, Romans vii: "I find in me, that is in my flesh, no good thing."

II. If this work of chastity is to be permanent, it will drive to many other good works, to fasting and temperance over against gluttony and drunkenness, to watching and early rising over against laziness and excessive sleep, to work and labor over against idleness. For gluttony, drunkenness, lying late abed, loafing and being without work are weapons of unchastity, with which chastity is quickly overcome. On the other hand, the holy Apostle Paul calls fasting, watching and labor godly weapons, with which unchastity is mastered; but, as has been said above, these exercises must do no more than overcome unchastity, and not pervert nature.

Above all this, the strongest defence is prayer and the Word of God; namely, that when evil lust stirs, a man flee to prayer, call upon God's mercy and help, read and meditate on the Gospel, and in it consider Christ's sufferings. Thus says Psalm cxxxvii: "Happy shall he be, that taketh and dasheth the little ones of Babylon against the rock," that is, if the heart runs to the Lord Christ with its evil thoughts while they are yet young and just beginning; for Christ is a Rock, on which they are ground to powder and come to naught.

See, here each one will find enough to do with himself, and more than enough, and will be given many good works to do within himself. But now no one uses prayer, fasting, watching, labor for this purpose, but men stop in these works as if they were in themselves the whole purpose, although they should be arranged so as to fulfil the work of this Commandment and purify us daily more and more.

Some have also indicated more things which should be avoided, such as soft beds and clothes, that we should avoid excessive adornment, and neither associate nor talk with members of the opposite sex, nor even look upon them, and whatsoever else may be conducive to chastity. In all these things no one can fix a definite rule and measure. Each one must watch himself and see what things are needful to him for chastity, in what quantity and how long they help him to be chaste, that he may thus choose and observe them for himself; if he cannot do this, let him for a time give himself up to be controlled by another, who may hold him to such observance until he can learn to rule himself. This was the purpose for which the monastic houses were established of old, to teach young people discipline and purity.

III. In this work a good strong faith is a great help, more noticeably so than in almost any other; so that for this reason also Isaiah xi. says that "faith is a girdle of the reins," that is, a guard of chastity. For he who so lives that he looks to God for all grace, takes pleasure in spiritual purity; therefore he can so much more easily resist fleshly impurity: and in such faith the Spirit tells him of a certainty how he shall avoid evil thoughts and everything that is repugnant to chastity. For as the faith in divine favor lives without ceasing and works in all works, so it also does not cease its admonitions in all things that are pleasing to God or displease Him; as St. John says in his Epistle: "Ye need not that any man teach you: for the divine anointing, that is, the Spirit of God, teacheth you of all things."

Yet we must not despair if we are not soon rid of the temptation, nor by any means imagine that we are free from it as long as we live, and we must regard it only as an incentive and admonition to prayer, fasting, watching, laboring, and to other exercises for the quenching of the flesh, especially to the practice and exercise of faith in God. For that chastity is not precious which is at ease, but that which is at war with unchastity, and fights, and without ceasing drives out all the poison with which the flesh and the evil spirit attack it. Thus St. Peter says, "I beseech you, abstain from fleshly desires and lusts, which war always against the soul." And St. Paul, Romans vi, "Ye shall not obey the body in its lusts." In these and like passages it is shown that no one is without evil lust; but that everyone shall and must daily fight against it. But although this brings uneasiness and pain, it is none the less a work that gives pleasure, in which we shall have our comfort and satisfaction. For they who think they make an end of temptation by yielding to it, only set themselves on fire the more; and although for a time it is quiet, it comes again with more strength another time, and finds the nature weaker than before.