A person who does not believe in God for what can only be one of two reasons:
1) They have not been told the truth that God sent his Son, Jesus Christ, down to Earth and sacrificed Him in order to save EVERY SINGLE PERSON to come after Him. They do not know yet that there has been a holy death in order to lift their sins from them and that they can be FORGIVEN eternally if they simply oper their heart to the love of Christ.
For God so loved the world that He gave His one and only Son, that whoever believes in Him shall not perish but have eternal life.
2) They are aware of the truth regarding Christianity and God but are simply too afraid of how to go about "becoming" a Christian
or what people will think of them if they do that they brush off religion as "unproven" and prefer to justify every teeny tiny amazing aspect of beauty on this planet as a work of "science" and pretend to feel content.
There is absolutely NOTHING that can fill the emptiness of the human soul completely except for Jesus Christ. DO NOT settle for being an Atheist when there is something truly so much better out there.