Creation. The order, variety, and beauty we see in nature are conclusive proofs that there is a Creator.
"For since the creation of the world, His invisible attributes, His eternal power and divine nature have been clearly seen, being understood through what has been made, so that they are without excuse." (Romans 1:20, NASB)
"The heavens declare the glory of God . . . There is no . . . language, where their voice is not heard." (Psalm 19:1,3)
Conscience. The inner voice that condemns us when we break God's law is evidence that there is a Lawgiver.
"For when the Gentiles, who do not have the law, do by nature the things contained in the law, these, having not the law, are a law to themselves: who show the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing them" (Romans 2:14,15)