17 When the world was complete and its inhabitant was to be created, the words spoken concerning him were, Let Us make man after Our image and likeness. I ask you, Do you suppose that God spoke those words to Himself? Is it not obvious that He was addressing not Himself, but Another? If you reply that He was alone, then out of His own mouth He confutes you, for He says, Let Us make man after Our image and likeness. God has spoken to us through the Lawgiver in the way which is intelligible to us; that is, He makes us acquainted with His action by means of language, the faculty with which He has been pleased to endow us.
There is, indeed, an indication of the Son of God, through Whom all things were made, in the words, And God said, Let there be a firmament, and in, And God made the firmament, which follows: but lest we should think these words of God were wasted and meaningless, supposing that He issued to Himself the command of creation, and Himself obeyed it—for what notion could be further from the thought of a solitary God than that of giving a verbal order to Himself, when nothing was necessary except an exertion of His will?— He determined to give us a more perfect assurance that these words refer to Another beside Himself.
When He said, Let Us make man after Our image and likeness, His indication of a Partner demolishes the theory of His isolation. For an isolated being cannot be partner to himself; and again, the words, Let Us make, are inconsistent with solitude, while Our cannot be used except to a companion. Both words, Us and Our are inconsistent with the notion of a solitary God speaking to Himself, and equally inconsistent with that of the address being made to a stranger who has nothing in common with the Speaker.
If you interpret the passage to mean that He is isolated, I ask you whether you suppose that He was speaking with Himself? If you do not understand that He was speaking with Himself, how can you assume that He was isolated? If He were isolated, we should find Him described as isolated; if He had a companion, then as not isolated. I and Mine would describe the former state; the latter is indicated by Us and Our.
Source: On the Trinity (New Advent)