I am familiar with the argument, but it is fallacious. It's known as the No True Scotsman fallacy.
No true Scotsman - Wikipedia
The fallacy works like this: "I once knew a Scotsman that drank Budweiser." Reply, "No true Scotsman would ever do a thing like that!"
Basically, the people who committed these crimes against the Jews were believers in Jesus, that he was the Christ and that he died for their sins, etc. They had been baptized into the church. They had not left Christianity for some other religion--they were Muslims or Hindus or Atheists. They were Christians.
Were they sinning? Of course. Were they devout, living their lives on the high level that their religion invites? Doubtful.
But they were still Christians.
It's not a whole lot different from some of the Christians (and Jews and Hindus and other people) participating in first the splitting apart of immigrant children from their asylum seeking parents, and now the imprisonment of those children with their asylum seeking parents. It's an obvious human rights issue, but people manage to go to church on Sunday and lock up kids on Monday.