Andrew Klavan and Bill Whittle discuss whether a conservative who lacks a belief in the existence of God is being consistent. Their answers are interesting and helpful, after the first thirty seconds of silliness, but I wonder if there's not more that can be said about this.
It's true that conservatives are often, though not always, theists, particularly Christian theists, but maybe it's equally as interesting to ask why Christians are usually (but certainly not always) conservatives. One reason for this correlation, perhaps, is that Christians believe in the inherent depravity of men. Thus, they tend to believe that when power is centralized in a big, bureaucratic government in which anonymous men and women are unaccountable to the people they putatively serve but whom they in fact rule, then all manner of evils ensue. Thus the Christian doctrine of man's fallenness fits snugly with the conservative predilection for small, accountable government.
Liberals and progressives, on the other hand, tend to see human beings as basically good and even perfectable. They believe that by creating better incentives, better environments, by freeing men from their inhibitions and guilt, we can construct a better world. Creating these conditions, they also believe, can best be accomplished by a strong central government. Like atheists, liberals tend to see the world in evolutionary terms, and the idea of progressively evolving toward something better than we are today resonates with them. Thus the atheistic belief in evolutionary advance fits well with the liberal belief that the state should be the engine of that progress.
Conservatives and Christians, of course, think the idea of state induced human improvement is a fantasy. Human beings are basically flawed. They're by nature selfish, violent, promiscuous, and power-hungry. Since that is their nature, changing their environment without changing their heart will only bring about marginal improvements in their behavior. Moreover, investing autonomous power in the government only insures that these human vices will be writ large in the state.
Conservatives also believe deeply in the social value and efficacy of the mediating institutions of family, church, school, voluntary associations, etc. and that government has no role in these except to prevent abuses within the institutions. This conviction dovetails neatly with the Protestant notion of sphere sovereignty and the Catholic idea of subsidiarity, which gives conservatives and Christians another point of resonance that is not so important to liberal statists. Indeed, liberals often place little value on these mediating institutions and the omnicompetent government they endorse is one which often seeks to squeeze them out of existence so that nothing remains between the individual and the state. The progressive statist wants the individual's greatest allegiance to be not to his community, his family or to his God but to his government.
Furthermore, both Christians and conservatives also believe that our rights derive from God, not from the state. Our rights to life, liberty, and the pursuit of happiness are not bestowed upon us by bureaucrats, they are bestowed by God and are thus unalienable. One who does not believe in God has nothing in which to ground unalienable rights, nor, for that matter, human worth and dignity. Christian conservatives believe with John Locke that, though man is fallen, he nevertheless is created in the image of God and is loved by God and therefore has worth and dignity. Atheist progressives believe that man is an animal in many important respects just like any other animal and is therefore nothing "special." His rights are arbitrary gifts bestowed upon him by the state because the state finds them expedient and are granted or revoked depending upon the current needs of the state.
So conservatives can be, and sometimes are, atheists, but the metaphysical basis for conservatism in the West is the Christian tradition. As was pointed out in the video, no nation without this tradition has ever placed a value on individual freedom and worth.
An atheist who embraces conservative principles, and there are many who do, does not do so because of the theistic ground upon which conservatism rests but because he or she simply believes conservatism works best to produce the ends he/she desires.