Religion invokes a One-sided World View

[Wyrd of Gawd]

If a person is religious at all it's stupid for him to believe in any other religions or to give them any credibility.

No, obviously he won't be believing in any other religions. However, it does create a gap of understanding.

He doesn't have to give them credibility either. For instance, as Atheists, we don't give any religion credibility, because there is no credible evidence behind any one of them. However, the difference is, we at least understand why people will choose those religions. Most religious peoples, however, fail to understand the reasoning behind people choosing different religions, and chances are if they did, they would start to question the reasoning that they use to justify believing their own religion, and come to the conclusion that there is nothing in particular that justifies believing one over the other. However, we are talking about the people who haven't questioned this, the average religious person does not usually stop to question or ponder over the authenticity of their belief, and for their own lack of understanding about the reasoning of their own belief, the biproduct is a lack of understanding of reasoning for anyone else's belief.

Religion is a lot like music appreciation.  People generally like what they were initially exposed to more than what they hear later on in life.