My understanding is: without religion, there would have been no holocaust.
Your understanding is mistaken. There were Christians who supported Hitler, and there were Christians who opposed him. There were Christians who hated Jews, and there were Christians who hid them & helped them escape. Hitler hated Christians, and would have eventually done the same thing to Christians as he did to Jews. Wikipedia actually has a pretty good description of his religious upbringing and the influence it had on him.