OK Mikel, I skimmed the exchange and got where he was coming from. Here's some things to consider.
Your opponent made a Logical fallacy in defining Christians.
Logical fallacy - There is No True Scotsman.
No Christian ever thought Jesus was anything but God
The Ebionites did not believe Jesus was God
Ah yes, but no TRUE Christian ever thought Jesus was anything but
The fallacy is in constantly modifying the definition of Christian to exclude anyone that disagrees with the initial premise. There were, in fact, several sizeable groups in the early centuries that considered themselves true Christians and followers of Jesus, but they did not hold to the God-Man concept later developed in Rome.
The Ebionites thought Jesus was just a man that was adopted by God as a special son (he had not been preexistent or part of any Trinity). They were well established by the early 2nd century.
The Marcionites, another sizeable 2nd century group, also considered themselves true Christians and followers of Jesus. They held that the vengeful god of the OT and the loving God of Jesus were 2 separate gods. They emphasizes Pauls "likeness of flesh" in Romans 8:3 to conclude that Jesus was not really a physical human, but a spiritual manifestation of God.
Both the Ebionites and the Marcionites were "labeled" heretical by the powerful church in Rome, but that doesn't mean they weren't Christians, only that they disagreed with the Roman Bishops.
In modern times, there are numerous groups that believe they have the correct understanding of who Jesus is (e.g. Judaism, Islam, JW, Mormons, etc.) and they disagree significantly with Christian fundamentalists.
"...are our..[Pagan beliefs] to be accounted myths and theirs [the Christians'] believed? What reasons do the Christians give for the distinctiveness of their beliefs? In truth, there is nothing at all unusual about what the Christians believe..." Celsus, late 2nd century CE, commenting on the similarities between Pagan and Christian beliefs.
While your opponent may assert that all these notions have been disproved a century ago, assertions are useless. He must present the solid, incontrovertible, evidence or admit the possibility of Christian borrowing from paganism.
Also, Darren sent me this response regarding the question of the past cruelty of Christianity:
In answer to Thomas Paine's question regarding when the cruelty of Christianity became apparent:
Following Constantine's conversion to Christianity.
The section regarding Constantine's conversion is quite revealing. Further down is a section about the new Rome and Constantine's government. Finally, there is this about Theodosius I:
"Theodosius' importance rests on the fact that he founded a dynasty which continued in power until the death of his grandson Theodosius II in 450. This ensured a continuity of policy which saw the emergence of Nicene Christianity as the orthodox belief of the vast majority of Christians throughout the middle ages. It also ensured the essential destruction of paganism and the emergence of Christianity as the religion of the state, even if the individual steps in this process can be difficult to identify. On the negative side, however, he allowed his dynastic interests and ambitions to lead him into two unnecessary and bloody civil wars which severely weakened the empire's ability to defend itself in the face of continued barbarian pressure upon its frontiers. In this manner, he put the interests of his family before those of the wider Roman population and was responsible, in many ways, for the phenomenon to which we now refer as the fall of the western Roman empire."
Thanks for your input!