**Oneness is impossible because Jesus prays to his Father in John 17 and many other verses show quite clearly two separate persons.**
You trinitarians admit that the Son of God was a man in every sense of the word, including having a soul. Oneness believe that as well. We simply believe that the ‘all power’ in him is God the Father.
“..I foresaw the Lord always before my face, for he is on my right hand, that I should not be moved: Therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope: Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption. Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance.” Acts 2:25-28
While those are words from the prophet David, is not about him. For one, David was not the Holy One. Also, David’s body stayed in the grave and saw corruption.
Oneness believe that the image of God is not God, but the Son of God, for God is a Spirit. We believe that the Holy One was the first creation of God the Father in the beginning, and was incarnated to save mankind.
**For example, at the baptism of Jesus.**
I can talk, drive a car, and engage a garage door opener all at the same time. Jesus Christ points out several times in chapters 14,15, and 16, that he is from the Father, and so is the Holy Ghost. What do you limit the Father to in spiritual ability? What things can he do at the same time, and what can he not do at the same time?
**The other huge problem you have of course, is the Church left behind by the Apostles believed in the Trinity.**
The apostles left behind the teachings that they were given by the Lord. As the Lord never used the phrases ‘God the Son’, or ‘God the Holy Spirit’, neither did they.
After the apostles died, there were men that, like the Pharisees, wanted to be the foremost teachers. Paul, John, Peter, and Jude warned of it being a present problem in their time. Consequently, there was plenty of push and shove about the spiritual understanding of the Godhead. We know that the devil loves deception and confusion; a little or a lot, but I’m sure he prefers a lot.
There is plenty of history to show the disagreements from approximately 100 A.D. onward. It was even brought to the forefront when the great John Calvin (cough) encouraged the execution of oneness witness Michael Servetus. At least (it is said) Calvin just wanted him beheaded, not burned at the stake like he actually was (isn’t that nice/sarc).
Why has oneness been small in number (in our opinion)?....well, there are any number of reasons. The biggest being the Lord’s own declaration: “Strait is the gate and narrow is the way, and few there be that find it.”
Paul wasn’t the only high-strung Pharisee; maybe the most zealous though. Why didn’t God literally blind a couple more around the same time, straighten them out, and send them into Asia (China, Japan, India, etc)?
God only knows. He has his reasons for doing things his way.
For us, they are past finding out.