How can you prove the absence of something? To definitively assert that something (or someone) does not exist when you lack complete knowledge of a subject is foolish! The Bible is quite plain on the subject, as this passage from the 14TH Psalms says.
Psalms 14:1-3 WEB (R)
(1) The fool has said in his heart, "There is no God." They are corrupt. They have done abominable works. There is none who does good.
(2) Yahweh looked down from heaven on the children of men, to see if there were any who did understand, who did seek after God.
(3) They have all gone aside. They have together become corrupt. There is none who does good, no, not one.
At least the agnostic is honest enough to say "I don't know". Consider the complexity of the universe or perhaps even our own bodies and it is extremely difficult to believe these things happened by mere chance. When you honestly look at yourself and the world around you, any explanation of origins will ultimately yield a bland page. To continue to assert this belief after seeing the truth just reveals how illogical and corrupt a mind can be. Its no wonder Paul quoted from the 14TH Psalms in the book of Romans (3:10 ff.).