The vikings didn’t just raid britain, they conquered and settled a substantial amount of it in the late 9th century, and later fully conquered england in the early 11th century (Swein, though he died before being crowned and his son Cnut held the thrones of England and Denmark, iirc). They had similar success in northern france as well. At the time this was going (9th ct) on they were so pagan that I have trouble imagining them having any interest in religious issues on the continent.
I have never seen a single reference anywhere on conversion events in saxony having anything to do with the beginning of viking raids. Vikings weren’t too picky about who they raided, they would raid across to other norse countries as well if it were permitted.
The theory is around. Houghton M. mentions it more fully than I did to discredit it. His sources might be a place to start if you’re interested.
No doubt the Vikings raided other heathens, and when surrounded by Christians and of a mind to raid, it would have been hard not to raid Christians. Their interest in religious issues on the continent, of which Denmark is a part, would have been related to watching one group of followers of Odin and Thor be mass murdered by their Christian neigbors, and wondering if that’s something to expect from Christian neighbors.
I don’t think the theory, or hypothesis, that there was a connection posits that there was an Odinist holy war against the Church of Rome, of which raids on coastal monasteries and churches were just a few battles.
I’m not 100% sure, but I thought Cnut was emphatically a Christian king. His son was supposedly much less devout (or perhaps more devoutly Wotanic), but not Cnut.