If you were created by a Creator, and that Creator wishes to have an ongoing relationship with you, then it is up to you to establish and maintain that relationship, should you so choose.
Proof, I believe, of God would come from the revelation of His unconditional love. If we discover that God is unconditionally loving towards us, then faith (optimistic conviction) would be established and a "father-son" relationship would grow over time.
Is that difficult to understand for an atheist? I wouldn't say the belief is on the same level as having an imaginary friend.