31 And thus this passage of prophecy, taken with its whole context, clearly describes as God both Him Whom we serve for the new name's sake, and Him through Whom the new name is blessed upon earth. It tells us Who it is that is blessed as true God, and Who is sworn by as true God. And this is the confession of faith made, in the fullness of time, by the Church in loyal devotion to Christ her Lord. We can see how exactly the words of prophecy conform to the truth, by their refraining from the insertion of that pronoun of the second person.
Had the words been You, the true God, then they might have been interpreted as spoken to another. The true God can refer to none but the Speaker. The passage, taken by itself, shows to Whom it refers; the preceding words, taken in connection with it, declare Who the Speaker is Who makes this confession of God. They are these:— I have appeared openly to them that asked not for Me, and, I have been found of them that sought Me not. I said, Here am I, unto a nation that called not on My name.
I have spread out My hands all the day to an unbelieving and gainsaying people</em>. Could a dishonest attempt to suppress the truth be more completely exposed, or the Speaker be more distinctly revealed as true God, than here? Who, I demand, was it that appeared to them that asked not for Him, and was found of them that sought Him not? What nation is it that formerly called not on His name? Who is it that spread out His hands all the day to an unbelieving and gainsaying people?
Compare with these words that holy and Divine Song of Deuteronomy, in which God, in His wrath against them that are no Gods, moves the unbelievers to jealousy against those that are no people and a foolish nation. Conclude for yourself, Who it is that makes Himself manifest to them that knew Him not; Who, though one people is His own, becomes the possession of strangers; Who it is that spreads out His hands before an unbelieving and gainsaying people, nailing to the cross the writing of the former sentence against us. For the same Spirit in the prophet, whom we are considering, proceeds thus in the course of this one prophecy, which is connected in argument as well as continuous in utterance:— But My servants shall be called by a new name, which shall be blessed upon earth, and they shall bless the true God, and they that swear upon the earth shall swear by the true God.
Source: On the Trinity (New Advent)