Every person knows deep inside that there is a God. Or do they? Theists believe there is a God, atheists believe there is not, and agnostics just don’t know.
So, we are faced with the question: is it possible for someone not to know there is a God?
Romans 1:18-20 makes the answer clear.
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, (Romans 1:18-20)
Every person is created with the knowledge that there is at least some kind of superior being in the universe. Whether or not that is the God of the Bible is beyond this discussion, but all we have to do is look around at nature and see evidence of God. We also can look inside ourselves and see a built-in morality, which also comes from God.
In nature, we see “natural revelation”–God revealing Himself through creation. However, to really know God, we also have to consider “special revelation”–the Bible, and Jesus Himself.