Mysterious Plurality of One God

"This triune God (or Trinity) began to allude to this aspect of His

nature right in Genesis 1:26–27. There we read that “God said, ‘Let us make man in our image’ . . . God created man in His image.” Here God is a plural noun, said is in the third-person singular verb form, and we see both the plural pronoun our and the singular His referring to the same thing (God’s image). 

This is not horribly confused grammar. Rather, we are being taught, in a limited way, that God is a plurality in unity. We can’t say from this verse that He is a trinity, but God progressively reveals more about Himself in later Scriptures to bring us to that conclusion.

@ In Isaiah 48:12–16 we find the speaker in the passage describing Himself as the Creator and yet saying that “the Lord God and His Spirit have sent Me.” This is further hinting at the doctrine of the trinity, which becomes very clear in the New Testament. There are many other Old Testament Scriptures that hint at the same idea.

@ In Matthew 28:18–20 Jesus' command His disciples to baptize His followers in the name (singular) of the Father, Son, and Holy Spirit. John’s Gospel tells us that “the Word” is God who became man in Jesus Christ (John 1:1–3, 14)." 

AIG