My sister believes in God (idk if she does about Jesus so maybe) but doesn't follow any religion. She used to be Catholic but now goes by no religion. Is she an atheist? Will she go to Heaven even if she doesn't believe in Heaven? She has been baptized and had her first communion.
If she believes in God, or any kind of god, she is not an atheist.
If one believes in God but doesn't conform to a certain religion, they are considered a Theist.
Actually, they would be considered a theist regardless as to whether they conform to a certain religion or not.
Or if they think there's a chance that there's a God or not and it's not for us to really know or have the desire to find out, then she's Agnostic.
Technically, an agnostic-theist.
So, to help make my point:
If the conditions of entering heaven is to believe in God, then yes, you HAVE to be a theist to get into heaven. You yourself are a theist. This doesn't necessarily mean all theists would go to heaven, since theism doesn't refer to a specific god. It only refers to whether or not someone believes in a god or gods.
So this response to your questions...
rom Christian standpoint, no...
... is just wrong. Because Christians ARE theists.
Atheist: someone who does not believe in any god and may or may not believe in the supernatural or spiritual
Agnostic: an atheist who is open to the possibility of there being a god or gods but does not currently believe in a god
Theist: someone who believes in a god or gods, but does not necessarily identify that god.
Christian: a theist who believes in the God of Abraham, Isaac and Jacob and who believes in the Gospel.
Hope that helps.
Actually, an agnostic is a person who doesn't know. An atheist who claims they don't know for sure there is no god would be an agnostic-atheist. It sounds like you sister is an agnostic-theist.
Atheists who claim to know there are no gods would be gnostic-atheists. People who claim they know for sure God is real are gnostic-theists.
Other than that discrepancy, TheArisocat hit the nail on the head.
Wat if she believes in God and Jesus but doesn't want to know anything about God's Word?
She is a theist, just like you and almost everyone else on this site. I would consider her a Christian, but most people on this site would argue she's not a "true" Christian. It depends on how you define Christianity. If she isn't a Christian, she leans towards Christianity.
The question isn't so much what you would consider her though. That's irrelevant. The question would be whether or not she can make it to heaven without looking more into the Bible. This is a question I am not at liberties to answer considering my own personal views.