I don't understand what you're trying to ask.

It seems that you're coming from the POV of automatically discounting Islam, making the assumption that the Prophet Muhammad (saws) made it all up and then asking for the source of his alleged story?

The Qu'ran states that Jesus (as) had a gospel of his own, but that gospel was lost and his message was corrupted. Islamic historians do place a lot of blame on Saul of Tarsus, due to the fact that he is the source of many Christian doctrines that are problematic from an Islamic perspective, and from a historical perspective his "back story" is almost certainly falsified in whole or in part. Christian Biblical scholars for the most part agree that there is a lost gospel, known as the Q source which contained sayings of Jesus (as) such as the Beatitudes, many parables, the Lord's Prayer and the Golden Rule, its believed that this served as one of the main sources for the Gospels of Luke and Matthew.

As for disbelief in the divinity of Jesus and the Nicene Triune Godhead, the Arian faction of Christianity preached that Jesus (as) was a man who was given divinity by God, but was not God himself. Arianism was very common amongst Germanic tribes such as the goths, and in Christian communities east and south of the Mediterranean, most notably Alexandria.

However, this isn't really a history question. It's a theological question made to give more evidence to an idea that you already consider to be true.