Getting to the heart of atheism rests on three questions. One, do you seek God? Two, could you recognize him if you found him? Three, would you worship him if you recognized him?
No one will find that which they do not seek. Thus, those who loudly proclaim God’s nonexistence with great certainty are not seeking God. It should thus come as no surprise that they have not found him. By the same measure, it shouldn’t be surprising if God has not revealed Himself to them. Their atheism is not honest, nor are they, and they should simply be ignored. They choose to disbelieve and disobey. You will not change their mind.
In keeping with this, no one will ever find that which they cannot recognize. It doesn’t matter how much time you spend searching for something if you do not know that for which you are searching. The atheist who proclaims himself a seeker—albeit an ignorant one—might merit some attention and education. Ignorance is certainly an understandable reason for not seeing God, especially since the adversary loves to deceive. Failure borne of ignorance is understandable, and even amenable, provided it is sincere.
Finally, those who refuse to worship God when in His presence should be surprised that He would not take pains to reveal Himself to them. What would be the point of presenting yourself to an ingrate? Thus, it should come as no surprise that those who would not worship God would also deny his existence.
If a man isn’t seeking God, couldn’t recognize him if were, and wouldn’t worship him if he did, then there is no reason to give him even the time of day. His disbelief is dishonest, and not amenable to evidence or reason. It is best to leave him be.