John Chrysostom

But if the old law had belonged to the devil, it would not have led people away from idolatry, but rather would have drawn them on and cast them into it; for this did the devil desire. But now we see the opposite effect produced by the old law. And indeed this very thing, the oath, was ordained of old for this cause, that they might not swear by the idols. For "ye shall swear," saith He, "by the true God." They were then no small advantages which the law effected, but rather very great. …

Homilies On The Gospel Of Matthew 17:5 ↗

Jerome

And you shall swear, 'As the Lord lives, in truth, in judgment, and in righteousness; and the nations shall bless him, and they themselves shall praise him.' And how does the Gospel forbid us to swear? But here (Matt. 5), to swear is said for confession, and for the condemnation of idols, by which Israel used to swear. Finally, stumbling blocks are removed, and one swears by the Lord. …

Commentary on Jeremiah ↗

Bonaventure

In the Old Testament it is said that he who swears in the name of God should swear and not perjure himself. For to perjure is the same as to forswear, and this is to take the name of God in vain. Jeremiah says: "And you shall swear: the Lord lives, in truth and in judgment and in justice;" and the Gloss says there that "an oath ought to have three companions: truth, judgment, and justice." First, an oath ought to be made with truth; that is, without falsehood. …

Collationes de Decem Praeceptis, Collation 3 ↗