36 The true method of our enquiry demands that we should begin with him, through whom God first manifested Himself to the world, that is, with Moses, by whose mouth God the Only-begotten thus declared Himself; See, see that I am God, and there is no God beside Me. That godless heresy must not assign these words to God, the unbegotten Father, is clear by the sense of the passage and by the evidence of the Apostle who, as we have already stated, has taught us to understand this whole discourse as spoken by God the Only-begotten.
The Apostle also points out the words, Rejoice, O you nations, with His people as those of the Son, and in corroboration further cites this:— And there shall be a root of Jesse, and One that shall arise to rule the nations; in Him shall the nations trust. Thus Moses by the words, Rejoice, O you nations, with His people indicates Him Who said, There is no God beside Me; and the Apostle refers the same words to our Lord Jesus Christ, God the Only-begotten, in Whose rising as a king from the root of Jesse, according to the flesh, the hope of the Gentiles rests. And therefore we must now consider the meaning of these words, that we, who know that they were spoken by Him, may ascertain in what sense He spoke them.
Source: On the Trinity (New Advent)