When Islam was first founded there were some Christians who considered it to be a heresy. 1
It wasn't until Mohammed was 40 years old that he claimed he was visited by the angel Gabriel and received his first revelation from God.
40 years is a long time, long enough to get around, maybe meet some Christians and learn about their doctrine, maybe even be baptized... Was Mohammed influenced by Christian ideology? Is it possible that he may have joined a Christian church before returning to Mecca and founding his own religion very similar to Christianity? Are there any theories about Mohammed once being a Christian?