The Christians have been divided and split on the basis of fundamental beliefs and theological issues from a very early point in their history. The theological divide between these Christian sects is huge.
Two early groups are worth note: the Ebionites and the Marcionites.
The Ebionites are known from sources dating back to the second, third, and fourth centuries after Christ. Second-century Christian heresiologist Tertullian says that the group is named after its founder, Ebion. Origen said that Ebionites come from the Hebrew word, ebyon, meaning ‘poor’. The Ebionites understood themselves to be Jewish followers of Jesus, i.e., Sayyidunā ‘Isā ‘alayhi as-salām. They observed the Sabbath, ate kosher, and circumcised all males. It is also reported that Ebionites faced Jerusalem when praying.
The Ebionites understood Sayyidunā ‘Isā ‘alayhi as-salām differently in comparison to Christians of their own time and Christians who came after them. They did not subscribe to the ideas of the pre-existence of Jesus or his virgin birth. The New Testament in its current form has these theological beliefs contained within it, but the Ebionites did not have the same gospels, epistles, etc., contained in the orthodox canon which became known as the New Testament.
The Ebionites believed in what is termed ‘adoptionism’, i.e., God adopted Jesus to be His son. They believed that Jesus was born as a normal person from his parents, but because he obeyed and adhered to the Mosaic law, God chose him to be His son. He then went to the cross, not as a punishment for his sins, but he sacrificed himself for the sins of the world. God accepted his sacrifice, and raised him from the dead, and took him up to heaven.
Ebionites believed in one God, unlike the Marcionites, who believed in two! The Marcionites lived around the same time as the Ebionites did. So, we have two early groupings of Christians who believed in almost two completely different theological systems.
When we contrast the Ebionites with the Marcionites, we find the following,
Marcionites believed in two Gods: One was the “wrathful” God of the Old Testament. The second was the merciful God who sent a spiritual entity named Jesus. Ebionites embraced the Jewish law, whilst Marcionites rejected it. Ebionites believed that Jesus was a man; not a God, whilst the Marcionites said that Jesus was a divine being, not a man. Marcionites held that Jesus only seemed human. Also noteworthy is that the Ebionites rejected Paul as an arch-heretic, whilst Marcionites felt that Paul was a true apostle.
Both the Ebionites and Marcionites were deemed heretics by the early Church fathers and “proto-orthodoxy.”
Interestingly enough, Epiphanius, a fourth-century bishop of Cyprus, passionately attacked the Ebionites. He stated that they (the Ebionites) held onto a now lost gospel. This gospel of the Ebionites was possibly written in Greek, (but their name is possibly derived from Hebrew) and Jesus spoke Aramaic. The language mix-up would be inevitable in their teachings, scripture, and gospel.
Which Christianity Is Authentic?
If you found a Christian from the first century, you would not know whether this Christian believed in one god or two gods, whether he believed in Jesus as man or divine being, whether he believed in the virgin birth or not, etc. On every major point of theology, there was vast divergence between Christian sects in the earliest period of Christianity.
This confusion of the Christian world leaves a Muslim with a sense of relief, because Allāh Ta’ālā, in His infinite wisdom, placed a system for the protection of the Noble Qur’ān in its pristine form, its language, vocalization, pronunciation, as well as its meanings are all intact and read today as it was by the final Messenger, Sayyidunā Muhammad Rasulullāh sallallāhu ‘alayhi wa sallam.
When we see modern Christian missionaries who are so passionate in their work, we should ask them how they can be so certain that the theology they preach is authentic. What proof do they have?
How can they be so sure they know the true teachings of Jesus when there were Christian sects living within only two or three generations after Jesus who believed that there were two gods or believed in adoptionism? And furthermore, these sects had their own gospels to justify their beliefs! Which gospels are authentic representations of Jesus’ message?
The Lost Christianities: The Battles for Scripture and the Faiths we Never Knew, Bart Ehrman, pp. 91-112.