Only One God

Who is God? Who are we? Who are we to believe? Why do we have God the Father, God the Son and God the Holy Spirit? Then why do we say, “There is Only One God?” Did God the Father mess up when He formed the Old Testament? Then why did God have to constitute a New Testament? Why did Jesus, the Son of God have to leave us? I can understand God the Father and the Son but who is this Holy Spirit? How are we created in the image of God? Why does Jesus say we are not of this world? Answers to these questions and more?