Justified Belief vs. Unwarranted Skepticism

Questions about whether it’s possible there’s a being that’s greater than God and that’s outside of God’s comprehension and omniscience, and how to explain to people who don’t believe in the spiritual realm or the existence of God that we have eternal souls.  

• Is it possible there’s a being that’s greater than God and that’s outside of God’s comprehension and omniscience?
• How do I explain to relatives who don’t believe in the existence of God or the spiritual realm that we all have an eternal soul? They say that if they see God do a miracle, then they’ll believe, but isn’t that why it’s faith?