Until the Romans got involved, Christianity's proportional make up had a high percentage of Jews, after all it was a Jewish sect for many years. Until the 4th century when the pretty exceptional event of being declared the state religion of Rome, replacing all others its main growth was not in Europe, but Syria, Egypt and the surrounding areas.
It also converted significant numbers of Zoroastrians in the early years, and it is from that tradition that concepts like angels were introduced into the Christian faith, Gnostic's were also common converts.
Even after the Roman conversion, it was centuries of work converting most of Europe. As with all things religion, look not always to reasons of faith and similarity for why conversion was quick or slow, but also to politics. The Orthodox Church for example was able to make inroads into Eastern Europe primarily based on their willingness to provide the liturgy in the native language of the people, while the Roman Church insisted on Latin.
When Islam first made its way to Africa many of the inroads Christianity had made were destroyed, and the native Christians converted to Islam. Egypt for example when from a mostly Christian nation under the Coptic Orthodox Church of Alexandria to a majority Islamic country by the 12th century.
I was going to dispute this post, but it actually is correct. I would say that conversion has more to do with circumstance rather than any closeness or distance between different religions. Also, Christian before the 4th century is actually a bit of a misnomer. There were so many widely conflicting variants, some that actually worshiped many gods, that considering it one religion at that point is ludicrous. The one thread that connected them at all is the inclusion of Jesus, and that's about it. From there the sky was basically the limit.
What many people are forgetting is that Rome made a habit of actively assimilating foreign religions into their own. The main reason that the romans clashed with early christianity had nothing to do with religious ideals, and everything to do with social and economic pressures. Many Christian sects refused to eat the meat of sacrificed animals. Since that meat was sold on the market, this hurt both the vendors of said meat and the temples who more than likely got a cut of the sales for upkeep purposes. The vendors, seeing a threat to their livelihoods, would put pressure on local governors to do something about the Christian sects that would not buy their cuts of meat. I could go on about this, but you see the point.
