John Calvin Commentary Matthew 23:18

"And, Whosoever shall swear by the altar, it is nothing; but whosoever shall swear by the gift that is upon it, he is a debtor." — Matthew 23:18 (ASV)

And whosoever shall swear by the altar. Here our Lord does what should be done in correcting errors, for He leads us to the source and shows, by the very nature of an oath, that the temple is far more valuable than the gifts that are offered in it. He accordingly assumes this principle: that it is not lawful to swear except by the name of God alone.

Therefore, it follows that whatever forms people may use in swearing, they must give to God the honor that is due to Him. And therefore it also follows how and to what extent we may 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 through such symbols of His presence, provided that He retains His authority unimpaired; for ascribing any divinity to heaven would be detestable idolatry.

Now, insofar as God presents to us a brighter mirror of His glory in the temple than in offerings, so much greater reverence and sacredness is due to the name of the temple. We now understand, therefore, in what sense Christ says that we swear by Him who inhabits heaven when we swear by heaven itself. His purpose is to direct all forms of swearing to their lawful end and object.