If I were trying to prove the existence of God to one who believes that there is no God, I would ask them the question, "If there is no God, then why does everyone know, deep down in their conscience, what is morally right and morally wrong?  Where did our conscience come from?"  I might, also, ask the question, "If there is no holy God, then by what standard do we measure right and wrong?"   Or, "If there is no God, then who designed the universe?" Or, "If there is no God, then why is there order in the universe?"  And, "If there is no God, what caused the universe to come into being?"  Finally, I might remind them that nothing is put into motion unless first put into motion by some outside force.  God is the first cause in this chain of cause and effect.  He is the creator.¹

Bible References:

¹ (Genesis 1:1)