I think it's certainly the best way to approach it, but equally, as long as he's aware that John isn't trustworthy and he should be careful, it might be for the best. That being said, if I were him, and someone wanted to force me to talk to a Catholic about God, I wouldn't be too happy.

You don't need to talk to me about religious cults. At all.

Anything I can do, I'll be quite happy to, because Costigan doesn't deserve a guy like John messing up his head about guilt and atonement. Which I imagine is his general theme, and really, nobody needs that.