It amazes me when people declare that they don't believe in God, and it is not for the reason people expect. When a person makes that declaration, what they are saying is they have chosen to not believe in the God they already know exists.

We all have a free will and we have the right to choose to believe whatever we want. We can even freely choose bad choices.
So how do we know that every adult already knows that God does truly exist?
We see the answer when there is a horrific or catastrophic event that is beyond what the person can cope with in their own strength. It is then that we see people cry out to God. There are a few individuals that even in those times they do not cry out to God, but they curse God. Either reaction is proof they do know that a God exists.
So the real question here is "Why would a person choose to believe something contrary to what they do truly already know?"
If we already know that there is a God, it becomes a matter of our choice between having Him in His rightful place or choosing to reject Him. It goes from a matter of belief to a matter of who do we want to be in control.
Are we going to take the time to figure out who God really is or are we going to reject Him as we don't want to give up our kingship over our own lives? If the latter, we probably already know that a God whose character is truth, love, and righteousness, would not approve of some of the things we are currently doing or have done.
People that have taken the time to find out who God really is, come to realize that God really is there to forgive them and bring about a higher quality of life for each of us. Some still reject God loving their own ways and this world more than what makes sense.
Over time our ways do catch up with us in the form of consequences.