"As may be expected, the phrase “Let us make man in our image” (Gen1:27) called for a lengthy commentary on God’s nature.
Luther sensed that this statement was intended to assure the reader of the divine mystery that “from eternity there is one God and that there are three separate Persons in one Godhead: the Father, the Son, and the Holy Spirit.” 
Being aware of various arguments proposed by Jewish scholars in objection to the Trinitarian interpretation of verse 27, he took time to critique those arguments
He concluded that the three divine persons cooperated in their creative activity. Therefore they express the deliberation of their council by saying “Let us make man” (v. 27).
The Father did not make one man, the Son another, and the Holy Spirit still another but “the Father, the Son, and the Holy Spirit, one and the same God, is the Author and Creator of the same work.”
Adam was not the image of only one of the three divine persons but of all three, signifying that he was made by “three separate persons in one divine essence.”
---Yet, the Father is only known in the Son and through the Holy Spirit. Thus God cannot be divided subjectively because he is one God “so far as His substance or essence is concerned. He is Father, Son, and Holy Spirit, three distinct Persons in One Godhead.”
DenisKaiser