Someone I know claims that the Catholic Church divided from the “Christian church” around A.D. 900. I know he is incorrect, but how do I explain it to him?
Tell him to go find a serious book on Church history and read it. There is no basis for what he is claiming. Not only were there no significant splits around A.D. 900, there were no splits anywhere near this period that resulted in a surviving group that was called “the Christian Church.”
The claim is so preposterous that one suspects your acquaintance has gotten something garbled somewhere along the line. If you really want to engage him on this subject, get more detail about what he is claiming. Insist that he give a source for the info, then research the claims made by that source.