This can stay in Forum Help, as this is a person asking for help, not a discussion about whether the username is bad or not.
Also, please don't speak for the staff.
Nevertheless:
In my opinion, it's fine. It's all about context really. 666 is just numbers, even with the religious aspect. As long as it's not offending anyone and you aren't using those numbers in a way to offend someone, I think it's fine. (Sorry I'm not sure how to really put this into words)
If it does become a problem for you or it begins making others uncomfortable in any way, the staff can change your name.
No worries!