Why do Muslims claim the Ebionites are proof Christianity was originally Islamic? The Ebionites had Jehovah's Witness tier beliefs not Islamic.
The Ebionites indeed did not believe Jesus was a pre-existent being, they believed he was only a righteous man who fulfilled the law of Moses perfectly and became the Christ because of this. However, the Ebionites distinguished Jesus from the Christ, sharing in common with the Nestorians that Jesus and the Christ were two separate persons.
In striking similarity with Jehovah's Witnesses, the Ebionites believe that the Christ was a super Archangel who created the universe on command of God and reigned over it as a second God; they believed that at the baptism of Jesus the Christ appeared as the Holy Spirit and completely infused himself into Jesus so as to completely transform Jesus into a divine being in a docetist or more accurately a monophysitic way and making him divine king over the whole universe. This is completely in contrast with the Islamic notion that Jesus Christ was a simply a prophet who was in no possible way divine.
Also, the Ebionites denied the virgin birth of Jesus, which again is against the Islamic view of Jesus.
If you see a Muslim bring up the Ebionites in a debate, bring up these facts.