18. And whosoever shall swear by the altar. Here our Lord does what ought to be done in correcting errors; for he leads us up to the source, and shows, by the very nature of an oath, that the temple is far more valuable than the gifts which are offered in it. He accordingly assumes this principle, that it is not lawful to swear but by the name of God alone. Hence it follows that, whatever forms men may employ in swearing, they must give to God the honor which is due to him; and hence also it follows in what manner and to what extent we are at liberty to swear by the temple, namely, because it is the residence or sanctuary of God; and by heaven, because there the glory of God shines. God permits himself to be called as a witness and judge, by means of such symbols of his presence, provided that he retain his authority unimpaired; for to ascribe any Divinity to heaven would be detestable idolatry. Now so far as God holds out to us a brighter mirror of his glory in the temple than in offerings, so much the greater reverence and sacredness is due to the name of the temple. We now perceive, therefore, in what sense Christ says that we swear by him who inhabits heaven, when we swear by heaven itself. His design is, to direct all forms of swearing to their lawful end and object.