How is it that everyone's cocksure of their own religion or spiritual path?

This question may not appear as exactly Buddhist or related to Buddhism but considering a bigger picture of Buddhism as a religion I want to know the answer to this. We Buddhists consider 'Doubt' in the dhamma or the Buddha as one of the hindrances to meditation, so most of us who do sincere practise are totally convinced with the Buddhist ontology and overall framework. So if we are true then definitely, the Advaita Vedantists or Christians or Islamists have something which is wrong. But considering the average human intelligence of practitioners of other religions, they do not find any thing wrong, in turn, people are hundred percent sure of their religion or spiritual practice. In fact, up to the last century, we were ready to die and kill each other for our religious beliefs. I have met some school students who go on preaching faith in the bible on roads, keeping their arguments aside, their conviction was admirable. I come from a Hindu background and most of the Hindu practices are about pleasing gods so that they will give their blessings or favor or some or other material boon. There is still so much poverty around but nobody seems to question the religion at large which is a major part of their lives. So to sum up my question, how is it that everybody is cocksure or their own religion?