Mother Teresa was a saint. I don't know that I could go for 50 years of doubting and feeling alone and still believe and act upon those beliefs. I would like to think I would, but I don't think you could know unless you'd done it. And to those people who say she must not have been a Christian, I say SHAME, SHAME, SHAME ON YOU.
Don't let anyone try to tell you what God sees in someone else's heart. He's the only one who knows. NO ONE has the inside scoop. No one but God has the right to make judgments. If someone tells you that Catholics, Orthodox, Mormons, Buddhists, etc., or any group of people or an individual or anyone is going to Hell, reject what that person says. They can keep their opinions to themselves.