Thomas Aquinas

Likewise, another reason follows: and whosoever shall swear by the temple, swears by it and by him that dwells in it. They said: he who swears by the temple swears by nothing. But he wishes to show that he who swears by the temple swears by God, because he does not swear by the temple except as sanctified, and it is not sanctified except by God. Therefore he who swears by the temple swears by God.

Commentary on Matthew ↗