At the outset, it is necessary for me to clarify that what I am about to discuss is not universally accepted among atheists. Because atheism is nothing more than the lack of theistic belief (i.e., an atheist is one who does not answer in the affirmative to the question of whether any sort of god or gods exist), the two prongs I will discuss are commonly held but not universal positions of the atheist. Atheists have many reasons for not accepting theism, and my selection of the two I will discuss here should not be mistaken as a suggestion that they are universal. I also acknowledge that an argument could be made that these prongs are more relevant to anti-theism than they are to atheism.
Prong 1: Faith is Irrational.
The function of thought is to promote understanding. For understanding to be possible, we must have some method of assessing the veracity of various claims. This method is reason. If I tell you that I believe in unicorns, you are in the position of evaluating the truthfulness of my claim. In applying reason, you expect me to provide evidence. After all, I'm the one making the claim. In your pursuit of the truth, you are likely to discover that my belief about unicorns is based on faith. Faith is required for me to maintain my belief because my belief has no justification, no supportive evidence.
Faith refers to the belief in something for which there is insufficient evidence to otherwise warrant belief. As George Smith demonstrates in Atheism: The case against God, the core of faith entails conflict with reason. Knowledge requires justification in the form of evidence; however, faith is belief in something without adequate justification. If evidence emerged that unicorns do in fact exist, faith would become unnecessary, even irrelevant.
Throughout time, many intellectually honest Christians have acknowledged that faith is the enemy of reason. Those who continue to insist that faith and reason are both valid ways of knowing must answer the question of why they need two ways of knowing when reason is sufficient for the rest of us. The inevitable answer, as Smith notes, is that the Christian wants to "claim as knowledge beliefs that have not been (and often cannot be) rationally demonstrated" (p. 104). Faith is the core of religious belief, and religion cannot exist without it.
Personally, I find this prong to be sufficient to reject the theistic claim about the existence of gods. If you ask me to explain why I am an atheist, I will inevitably mention the lack of evidence and the resulting irrationality of faith in its absence. Still, I do not find this prong sufficient to stimulate a drive toward secular activism. Demonstrating that religious belief is irrational is not the same thing as demonstrating that it is harmful. Thus, we come to the second prong.
Prong 2: Religion is Harmful.
This is a massive topic which will already be familiar to most atheists. That a number of bloody atrocities have been committed in the name of religion should not be surprising when one examines the contents of the "holy" tests which form the center for these religions (see this post at Ebon Musings for many examples from the Christian bible). However repulsive one finds these acts at which religion is the root, it would be a mistake to restrict our focus to bloodshed.
In the interest of brevity, I will leave it to others to catalog the many negative effects of religion. Instead, I'd like to keep this post as simple as possible by borrowing from Richard Dawkins' excellent The God Delusion. Whatever else religion is, I wholeheartedly agree with Dawkins that it is inherently divisive. He argues that religion divides us in ways that other types of human differences (e.g., nationality, politics, etc.) generally do not. Examples include labeling children, segregating schools, and taboos about marrying out of the in-group. The effects of this divisiveness have been widely documented and do not need to be repeated here.
While neither of these prongs must be accepted for one to be an atheist, their combination explains my atheism (prong 1) and motivates my interest in secular activism (prong 2).
This post was initially written in 2007. It was edited in 2018 to fix typos, update links, and improve clarity.