You must have learned in school that Christianity, Judaism, and Islam are all considered Abrahamic religions, which have influenced each other throughout history. They often share similar stories, figures, and values. All three religions include the story of Noah in their holy books. In these traditions, God commands Noah to build an ark to save his family and pairs of animals from a great flood sent to cleanse the Earth of wickedness. While there are variations in the details, Noah's Ark is certainly not exclusive to Christianity, it predates Christianity and is an important narrative in Judaism and Islam as well.
The guy sounds like a fruit cake. Germany was warned by Saudi Arabia about him but they ignored it.
Quite possibly he was anti Islam but he grew up and was influenced by a culture that openly hated Christians, as stated his X. Apparently there is a BBC documentary about this guy and how he helped asylum seekers to get to Europe. The optics aren't helpful and this will bring more support to AFD. German voters won't care what this attacker's belief system was, they will care that he murdered and maimed over 100 innocent people who were enjoying themselves at a Christmas market with family and friends and that he brought with him dangerous non western attitudes resulting in murdering the very people who helped him flee a regime he sought to get away from. He is one messed up and ungrateful individual.