Sincerity is not the way to heaven. What if you are sincerely wrong? If you are relying on your sincerity, then are you saying that because you are sincere...
therefore you are good enough on your own to be with God." Don't you see to appeal to your sincerity is to appeal to pride, because you are appealing to...
something that is in you and not God for your reason to go to heaven. Does this seem reasonable to you?
If the person asks where's the proof for God/Jesus/Christianity, go here.