Irrationality is the act of forming and maintain a belief without evidence.
People who believe in one thing for no reason (e.g. an omnipotent, omniscient, and omnibenevolent god exists) are likely to believe other things for no reason (e.g. Barack Obama is a socialist). Some of these baseless beliefs are benign, but others are highly dangerous, both to the individual and to society at large, if believed by enough people.
Belief in a god is not the cause of our problems, it’s only a symptom. Specifically, of a way of forming and maintaining beliefs without evidence. That method of belief formation seems to be the root cause of many social problems.
There also seems to be a link between conservative political positions and religion in terms of the emotion of fear. Both belief systems appeal to fear to gain adherence. Other similarities include simplistic world models, appeals to emotion, and weak requirements for evidence.
I am not anti-religion per se, I am anti-irrationality. But irrationality in one area often seems to spill over into irrationality in others. If belief in God and religiosity harmed no one, then I would have no problem with it. But it does cause harm, to certain groups (e.g. indoctrinated children, homosexuals, the believers themselves). Worst of all, many religious people vote for conservative Republicans, whose beliefs and policies do real harm to large numbers of people, both in the US and other countries (e.g. environmental damage, health damage, foreign relations damage, economic damage, educational damage, etc.). In a democracy, it matters what people believe because they can vote. Therefore, I have a vested interest in making those beliefs as true as possible.