The existence of God, this question is something that we think about whether we are believer or not. The bible tells us God created the heavens and the earth. We can know God through His creation. We can know God because his creation is there.
Romans talk about through the creation of God, there is deep sense in our heart, there is recognition of God's existence. Romans talks about what we do as mankind is to choose not retain the knowledge of God. Why don't we retain in the knowlege of God?