7 This being the case, I ask how a distinction can be made in the words, Let Us make man after Our own image and likeness, between a true God and a false. The words express a meaning, the meaning is the outcome of thought; the thought is set in motion by truth. Let us follow the words back to their meaning, and learn from the meaning the thought, and from the thought attain to the underlying truth. Your enquiry is, whether He to Whom the words Let Us make man after Our own image and likeness were spoken, was not thought of as true by Him Who spoke; for they undoubtedly express the feeling and thought of the Speaker. In saying Let Us make, He clearly indicates One in no discord with Himself, no alien or powerless Being, but One endowed with power to do the thing of which He speaks. His own words assure us that this is the sense in which we must understand that they were spoken.
Source: On the Trinity (New Advent)