Christians don't determine who lives with God for eternity, and they're not trying to. All they're doing is pointing out what Scripture says about sin. And what is your prescription then for when you stand before God and He judges you?
If Christians don't determine who gets into Heaven, then why is it most of them sound like they do? I've been told that I'm going to hell for many things, all the way from drinking, to having sex outside of marriage, to reading dirty magazines, to cussing, etc. A lot of them have told me that unless I change my life and repent of my sins, I'm going to hell (and some of them are pretty freaking judgemental when they say it).
What is my prescription for standing before God when I'm judged? Simple......admit where I went wrong, and then tell Him what I've done to correct my behavior since. In Jewish theology (the daddy of Christianity), they say that a person who has sinned and repented is held higher in Heaven than someone who has never sinned. Why? Because God doesn't want drones, and He wants us to use our free will to find out about this world. If we screw up and then repent and change our behavior, God likes that better, because then He is able to forgive.
I only think things that are "sins" are those things that are outlined in the 10 Commandments, or the 7 Noahide Commandments (both say basically the same thing). Anything else? Pretty much fair game, even if it is a vice, but you have to watch out when you play with vices, because that is where the temptation to sin hides. No average person is looking for ways to murder, steal and lie, because they know those things are bad, but if you follow a vice (such as drinking and you drink too much) you can eventually fall into sin. Drinking in and of itself isn't a sin, but if you lose your senses and become drunk, you might do something that is sinful.
Besides..........................Christians aren't the only ones with a lock on "Heaven". Many other belief systems have some version of Paradise as well, and that means to me that God is too big to be contained in just one religion, dogma, or belief system.
And..................didn't Jesus Himself say that there were many rooms in His Father's mansion?