Augustine of Hippo

"Israel, if you shall have heard Me, there shall not be in you any new god" [Psalm 81:9]. A "new god" is one made for the time: but our God is not new, but from eternity to eternity. And our Christ is new, perchance, as Man, but eternal God. For what before the beginning? And truly, "In the beginning was the Word, and the Word was with God, and the Word was God." [John 1:1] And our Christ Himself is the Word made flesh, that He might dwell in us. [John 1:14] Far be it, then, that there should be in any one a new god. …

Exposition on Psalm 81 ↗

Jerome

There shall be no new god in you. He whose god is his belly, a new god is his. As many vices as we have, as many sins, so many new gods do we have. I have been angered: anger is a god to me. I have seen a woman, and I have lusted: lust is a god to me. For each one, whatever he desires and reveres, this is his god as well. The greedy man has gold for his god. Nor shall you adore a strange god. Our God is virtue; the god of others, vices and sins.

Homily 13, on Psalm 80 ↗

Bonaventure

It must be said that this truth, that God is one, is a truth not only believable, but also intelligible: since it is necessary and certain not only from the testimony of Scripture and the illumination of grace, which is found in faith; but it is also certain from itself and from the testimony of creatures. From itself, therefore: because the divine being, on account of its singular sublimity and sublime singularity, possesses unity in every way. …

Disputed Questions on the Mystery of the Trinity, Question 2 ↗