A point of no return?
Just look at Saul/Paul. He was obviously an enemy to Christianity--doing everything in his power to stop people from accepting Christ. No doubt he committed other sins along the way, and yet he became one of the most powerful prophets in the Christian faith.
But maybe what we're talking about here is people who believe in Christ and turn away. From personal experience, I would hazard the guess that if you really actually believed in Christ, then you wouldn't turn away. I don't think people who have real faith can turn their backs on their religions.
One of the things that always irked me when I heard it was that there were only two sins that were nearly unforgivable... mortal sins. They were murder and adultery. I think murder and adultery can have a similar emotional affect on thier victims. Yet people "find God" after committing these sins all of the time.
I think that just leaves us with the sociopaths. Basically the people whose anti-social disorder leaves them entirely uncaring about society and the people in it. (This might just be too many episodes of Law & Order talking.) But... if it's a disorder, then is it really their fault?
Basically, I think Christianity's got a lot of loopholes... ways to allow people to come to Christ. This may have come off as cynical and negative, but my point is that the strength of Christianity is that there isn't a point of no return. Christ will always love you.