I guess I'd be surprised if an atheist or non-Christian used the name Christian. Not shock-and-amazement level surprised, mind you, because it is a common name. But it might just raise my eyebrows a bit, and I'd wonder why they chose a name with such an obvious connotation. That doesn't sound like it really describes your situation, though, so I don't think it would be strange for you to use it. There are probably a ton of Christians out there whose parents have a Christian background but are currently not-practicing.
Oh, FYI, Catholics are Christians, and there is more than one St. Christian, so Catholics use the name as freely as anyone else.
Christian Rafferty
Christian Malcolm
Christian Henry