Somebody said to me that he had been trying to share Christ with somebody who claimed to be an atheist and seemed to be very hard hearted. While I don't believe there's really a way to truly know somebody else's heart, there are certain clues as to the general state of somebody's attitude toward Jesus.
I asked him how the person he was sharing with reacted to conversational mentions of Jesus. He responded by telling me that the guy tended to get agitated and angry. Well, the guy might think he's an atheist, but from my experience, he's not that hard hearted. He's still on the soft side, because he can be easily touched. The people with whom you share Christ that react with nonchalance when you give them the Words of life, or who even joke with you about it, but never appear to be upset are the ones who's hearts are most likely hardened toward Him. In either case, whether the person who claims to be an atheist reacts angrily or passively to the name of Jesus, they are resentful of God for some reason of their own.
God gets blamed for much that He isn't responsible for. Someone loses a child so they blame God for taking them. Their life doesn't go the way they want it to, so they blame God. A 'Christian' did them wrong, so they blame God. The reasons are just as varied as they are ridiculous.
It usually boils down to an attitude. "I'll show You God. I won't believe in You. So there."
Sometimes a witness to an 'atheist' is a matter of establishing a bond of trust, revealing ourselves to them over time, and if we have the opportunities, to let them see Jesus in us.