"All you have to do get into heaven is believe that Jesus existed." That's the sort of thing you'll hear when you walk into churches around the US; it's not true.
Of course the only technical requirement to getting into heaven is to have Faith in Jesus, but Faith requires so much more than believing that Jesus existed. It requires belief; yes. More than that, it requires acting upon that belief.
Think of what James 2:19 says: You believe there is one God. Good! Even the demons believe that; and shudder.
So maybe it isn't only 'belief' that saves a person's soul.