I'm with wz here. There is a difference between saying someone is a hypocrite and committing and "un-Christian" act, and saying they don't have a right to call themselves Christian. The problem with the latter is that, in the end, our opinion is worthless because God decides whose worship He accepts. Not only are we opining on something we know very little about, but we run the risk of speaking for God when we have no right to do so.
It is much more precise and to the point, I think, to judge the act rather than the person as a whole.
There's a story in the Islamic tradition of a woman who was committing adultery. One day while walking outside she encountered a thirsty dog, and stopped to draw water from a nearby well and used her shoe to give the dog a drink. For this very small act of mercy God forgave her crime.
I've always really liked this story, in part because it emphasizes that we don't know -- can't know -- the balance of a person's soul. I'd rather run the risk of overlooking a crime than risk condemning a person who doesn't deserve it. If we believe that God is perfectly Just, well then no one can escape their due punishment. But it is possible in trying to condemn that we needlessly cause injustice, and then how have we helped anything?