Contrary to popular opinion, atheism does not exist.
Atheism is defined as that group of beliefs that deny the existence of God (or gods). In similar manner, Christians do not believe in the existence of atheism--at least not in principle. Romans 1:18-19 explains that the truth of God is revealed to all creation, to all mankind:
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. (NKJV)
The judgment of God the Creator is revealed, manifested and otherwise shown to mankind. In particular, the judgment against ungodliness is revealed to men. And these very same people "suppress the truth in unrighteousness". In fact everyone outside of Christianity suppresses this truth in manifold ways.
In other words a dual psychology is at work. Men and women both know and do not know the existence of God and the coming judgment. How? In very much the same way that a person does not truly know himself until the pressures of circumstance squeeze out surprising thoughts and actions.
Someone may have a mellow personality, thinking he is always calm and collective. But under the right circumstances and people, the calmness melts into anger. Similarly, many students prepare for a test only to find out how little they really know after staring at a blank page.
Thus atheism, as far as the Bible is concerned, does not exist. People may claim they deny God's existence, but on a more fundamental level they are aware of such a Creator. What this entails for the Christian defending his faith is a confidence that he is not speaking in a vacuum. It also helps explain the irrepressible human drive to be explicitly religious.
Yet without repentance from sin and faith in the perfection of Christ, such atheism is another way to avoid God--an avoidance without excuse. As Romans 1:20 reiterates:
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...