Positive and Negative Atheism
Negative atheism rests on the rejection of specific theistic beliefs; thus atheists through the ages have rejected the Greek and Norse pantheons, the afterlife, the divinity of Jesus, and the very existence of God. But there is also the sort of atheism which comes from positive assertions about the nature of the world based on observations and interpretations that simply don't involved God; this is the atheism of Darwinism, the Big Bang, cultural relativism, and reductionist materialism. Note that I don't mean to distinguish two types of atheist, but two types of atheism; an individual atheist will both disbelieve in specific religious assertions and believe in certain natural philosophies.
Negative atheism is the easier sort to overcome in favour of religious belief. Christian faith is more than believing in improbable miracles and inexpressible mysteries; theology offers rational, well-constructed arguments for God's existence, God goodness, for the necessity of evil, for Christ's incarnation and death, and for the muddle of baseness and nobility that is humanity. And where specific doubts persist, Christian ritual and spirituality which speak to our everyday experiences and perceptions can shore up a flagging faith. And behind reason and experience, religious belief is supported by the seemingly universal human need to believe in something.
Positive atheism, being not simply the lack of a theist worldview but providing rather an alternative philosophical system, offers a more serious challenge to the threat of religious conversion. Religous belief in the face of this sort of atheist requires not merely suspension of disbelief or acceptance of doubt, but rather the overthrow of whole systems of acceptable, rational ideas about what the world, the universe, and humanity really are.
I believe that it is the recent ascendency of postitive forms of atheism that has secured its modern currency, after centuries of near-universal religous faith.
Up until the 19th Century, much of what was considered by contemporaries to be atheism was really more of a partial atheism or agnosticism. While thinkers from Thales of Miletus in the 6th century BC to John Stuart Mill some 2,400 years later rejected the popular religion and gods of their age, they generally stopped short of completely rejecting the idea of God, still willing to concede or even requiring God as a prime mover or distant divine intelligence. Such philosophies are better considered forms of deism or panthesism.
The strength of modern atheism is that it is founded more upon positive atheism than negative; rather than just a rejection of contemporary religious beliefs, modern atheism offers a seemingly complete philosophy of the world around us. The contemporary athiest not only sees much that is dubious in Christianity, but sees much that is redundant and just less satisfying than what is provided by modern education, science, anthropology and material culture.
Another key to the wide-spread acceptance of atheism is the nature of modern, Western life and society. Religion is more than a set of explanations to which we do or do not ascribe intellectual assent; religion touches, shapes and responds to far deeper, less rationally defined needs and desires. It is arguable that Christianity, as well as other traditional religions, do not relate to the inner desires and needs felt by the average, educated member of our society. Where once people faced death of family and friends early and often, mortality is now only vaguely real to most people until they are well into their adulthood. The world in which the bulk of society was subject to the will and whims of the few, powerful rulers and the brute forces of nature seems a relic of the past; we haven't the acute need for an afterlife to redress the wrongs of earthly existense, and we don't need beatitudes and Heavenly love to give dignity to a life of slavery and drudgery. It has been argued that Christianity is a slave's religion, suited to a slave mentality, and that modern, educated people are not only inclined by circumstance to shake off this outmoded worldview by are commanded to do so by honour and human dignity.