Unfortunately, it's not enough to simply believe that God exists.
For example, the Bible says in James 2:19, "You believe that there is one God. Good! Even the demons believe that—and shudder." Obviously, demons do NOT get to go heaven.
To enter heaven, a person must genuinely accept Jesus as their savior. When they do this they are spiritually reborn into God's spiritual kingdom in preparation for entering heaven when their body dies.
The word "salvation" is used to express God's provision for our sin. Salvation makes a person an heir of God with new rights who is justified before God in Christ.
So a person accepts Christ and, as a result, are justified before God and sealed with God's Holy Spirit in preparation for heaven.
Does this make sense to you?
My sister told me that she is a Theist but when I mentioned christian-theist she said no. I think she said no mostly because I said christian and she doesn't pick a religion. But she believes in God but I don't know about Jesus but I'll say most likely. Will she go to Heaven since she believes in God?