I recently had someone tell me that they are probably going to heaven when they die because they are a good person and they believe in God. In their mind (not only theirs -- this is a common belief held by many), good people go up and bad people go down.
According to the Bible, good people don't go to heaven (including those who believe in God, even Jesus), only those people who realize that they are wretched sinners and have been forgiven by God through Jesus' finished work on the cross at Calvary by repentance and faith in Christ alone.