And again, in answer to Question 109, whether in the seventh commandment God forbids only adultery, and such like gross sins, the Catechism teaches: “Since both our body and soul are temples of the Holy Ghost, he commands us to preserve them pure and holy: therefore he forbids all unchaste actions, gestures, words, thoughts, desires, and whatever can entice men thereto.” All the emphasis of this Lord’s Day, therefore, falls on the Christian virtue of chastity, rather than on marriage and adultery. 

Ursinus in his “Commentary on the Heidelberg Catechism emphasizes the same thing. Writes he: “God in this commandment enjoins and sanctions the preservation of chastity and marriage and hence authorizes marriage itself; for whenever God forbids anything, he at the same time commands and authorizes the observance of that which is opposite thereto God, now in this commandment forbidsadultery, which is a violation of conjugal fidelity. When God singles out adultery as the most shocking and debasing vice of all the sins which are repugnant to chastity, he at the same time prohibits and condemns all wandering and wanton lusts, whether they be found in married or unmarried persons, and prohibits all other sins and vices contrary to chastity, together with their causes, occasions, effects antecedents, consequents etc. And on the other hand, he enjoins all those virtues which contribute to chastity. The reasons of this are these: 1. When one thing is specified, all those are understood which are closely allied or connected with it. Therefore, when adultery is prohibited, as the most shocking and debasing form of lust, we are to understand all other forms of lust as forbidden at the same time. 2. Where the cause is condemned, there the effect is also condemned; and where the effect is condemned, there the cause is condemned. Hence the antecedent as well as the consequence of adultery are here forbidden and condemned. 3. The design of this commandment is the preservation of chastity amongst men, and the guarding of marriage, or keeping it holy. Whatever, therefore, tends to the preservation of chastity, and the protection of marriage, is enjoined by this commandment, whilst that which is opposed thereto is forbidden. 

In the same commentary Ursinus also defines or describe chastity as follows: “Chastity, in general, is a virtue contributing to the purity of body and soul, agreeing with the will of God, and shunning all lusts prohibited by God, all unlawful intercourse and inordinate copulation in connection with all the desires, causes, effects, suspicions, occasions, etc., which may lead thereto, whether in holy wedlock or in single life. The term chastity comes, according to some, from the Greek kazoo, which means to adorn, because it is an ornament, both of the whole man, and also of all the other graces or virtues. The name has, therefore, been given to this virtue by way of pre-eminence, inasmuch as it is one of the principal virtues which constitute the image of God, according as it is said, God is chaste, and will be called upon by those who are of a chaste mind, and has regard to such prayers.” 

Ursinus then distinguishes between two kinds of chastity, one of single life and the other of holy wedlock. And as the cause of chastity he mentions first the command of God; secondly, the preservation of the image of God; thirdly, the desire to avoid marring the image of God, and the union between Christ and the church: and finally, rewards and punishments. 

I would define chastity as that Christian virtue according to which, through the grace of God in Christ Jesus our Lord, we preserve our soul and body pure, that is, free from any form of fornication or sexual lusts, whether in holy wedlock or in single life, and that too, from the principle of the law of God written in our hearts, to love the Lord our God with all our heart and mind and soul and strength, and our neighbor as ourselves.

This is the teaching of the Heidelberg Catechism in Lord’s Day 41 concerning the seventh commandment. All uncleanness, all that is not chaste, is accursed of God. With all our heart we must detest this uncleanness, and live chastely, whether in holy wedlock or in single life. Our body and soul are temples of the Holy Ghost. We must preserve them pure and holy. Everything that is not chaste is condemned by this seventh commandment, whether unchaste actions, gestures, words, thoughts, desires, or whatever can entice man thereto. And what shall we say as Christians, as people of God, concerning these things? 

And what shall be our attitude over against the seventh commandment? Shall we exalt ourselves, boasting that we are delivered by grace from all this corruption, and that to listen to the prohibition of the seventh commandment is beneath our Christian dignity? That would indeed be very foolish. To assume such an attitude would manifest that the Christian, or he that calls himself a Christian, does not know himself. O, it is true, grace delivered him in principle from all sin, also from the corruption of adultery and of all uncleanness. Yet the Christian knows that after all he has but a small beginning of the new obedience. He is indeed delivered in principle, but the motions of sin,—also of this sin against the seventh commandment,—are still in his flesh. He hears the word of the Lord that he who but looks on a woman to lust after her, has already committed adultery with her in his heart.