42 As you have listened already to Moses and Isaiah, so listen now to Jeremiah inculcating the same truth as they:— This is our God, and there shall be none other likened unto Him, Who has found out all the way of knowledge, and has given it unto Jacob His servant and to Israel His beloved. Afterward did He show Himself upon earth and dwelt among men. For previously he had said, And He is Man, and Who shall know Him? Thus you have God seen on earth and dwelling among men.
Now I ask you what sense you would assign to No one has seen God at any time, save the Only-begotten Son, which is in the bosom of the Father, when Jeremiah proclaims God seen on earth and dwelling among men? The Father confessedly cannot be seen except by the Son; Who then is This who was seen and dwelt among men? He must be our God, for He is God visible in human form, Whom men can handle. And take to heart the prophet's words, There shall be none other likened to Him.
If you ask how this can be, listen to the remainder of the sentence, lest you be tempted to deny to the Father His share of the confession. Hear, O Israel, the Lord your God is One. The whole passage is, There shall be none likened unto Him, Who has found out all the way of knowledge, and has given it unto Jacob His servant and to Israel His beloved. Afterward did He show Himself upon earth and dwelt among men. For there is one Mediator between God and Men, Who is both God and Man; Mediator both in giving of the Law and in taking of our body.
Therefore none other can be likened unto Him, for He is One, born from God into God, and He it was through Whom all things were created in heaven and earth, through Whom times and worlds were made. Everything, in fine, that exists owes its existence to His action. He it is that instructs Abraham, that speaks with Moses, that testifies to Israel, that abides in the prophets, that was born through the Virgin from the Holy Ghost, that nails to the cross of His passion the powers that are our foes, that slays death in hell, that strengthens the assurance of our hope by His Resurrection, that destroys the corruption of human flesh by the glory of His Body.
Therefore none shall be likened unto Him. For these are the peculiar powers of God the Only-begotten; He alone was born from God, the blissful Possessor of such great prerogatives. No second god can be likened unto Him, for He is God from God, not born from any alien being. There is nothing new or strange or modern created in Him. When Israel hears that its God is one, and that no second god is likened, that men may deem him God, to God Who is God's Son, the revelation means that God the Father and God the Son are One altogether, not by confusion of Person but by unity of substance. For the prophet forbids us, because God the Son is God, to liken Him to some second deity.
Source: On the Trinity (New Advent)