The question is, are they worshiping the true God?
The person who rejects the God of the Bible is a religionist, they're just denying The God they know exist by suppressing the truth in unrighteousness. So it doesn't matter whether they call themselves an "atheist" or agnostic (which is defined as ignoramus in the dictionary), they're still worshiping something. Instead, this person worships the person they see in the mirror, "the man in the mirror" -- or the person they see in the mirror. We are to pray and reason with people through God's truth so that God might perhaps lead them to repentance.
Repentance is a grace of God. One can only truly repent if God leads them towards the truth. God has revealed truth to all of us -- because we all know He exists and He has placed the truth of right and wrong within our understanding AND and everything around us declares God's glory.
When man denies God, it's because they're runners... people running from the truth, and they will use anything, even absurdity to get away.