EWTN Catholic Q&A
Re: Book of Genesis using plural references for God
Question from David McMahon on 12-24-2012:

Dear Dr. Geraghty,

I recently attended a Bible study where this same question was asked, and the teacher explained that "...let us make man in our image..." was a conversation among the Blessed Trinity.

Answer by Richard Geraghty on 12-25-2012:

Dear David,

Yes, it was a conversation among the persons of the Holy Trinity. But the writers of Genesis could not have known about it because it was not yet revealed. Nevertheless the expression they used left the way open to be interpreted as the Holy Trinity after it was revealed. God after all is the inspirer of the men who wrote the Old Testament.

Dr. Geraghty