Respect for religion is an entirely psychological phenomenon and it is easy to deduce why we 'have to respect' religious beliefs. Generally speaking religious beliefs touch the heart and core of the most cherished ideas mankind has ever invented, 'purpose', 'immortality through the soul', 'meaning', etc. Fear of death and annihilation is one of the key elements, since by telling someone his beliefs are nonsense you are in effect telling him that the immortality (one of the common reasons people adhere to religion) his religion promises is not going to happen and that death is final.
Meaning and purpose is yet another one. Objectively seen the universe lacks any specific purpose and certainly none for our species; most religions claim that the universe not only has purpose but that WE are the central object of that purpose, usually in conjunction with some father deity figure who invigilates above us in the 'heavens'.
A third factor is authority. In Western Europe where Christianity is weak, few people take umbrage at religious criticism, whereas the USA where religious nuts abound left and right and in the more extreme cases of the Gulf states, criticism can earn you the wrath of society at the very least and in some extreme cases the death penalty.
I contend that on a personal level points 1 and 2 are the most relevant. By telling someone his beliefs are bunk you are stripping him of his identity, his reason for living, without which he cannot live and upon death shall no longer live anyway.
Point 3 generally applies to the masses. In places like the USA religious leaders still have far too much authority and garner too much respect.
The important thing to remember is that how all of this goes hand in hand.
Whilst some of us would ostensibly 'respect' an indignant man blaming Hurricane Katrina on a lack of piety towards Yahweh the fertility deity that some believe is the creator of the universe, another man (rightly so) attributing the maritime disaster to ignoring Poseidon, Greek god of the sea and earthquakes (and thus a much more likely suspect) would be laughed off the pulpit or any other public places of speaking. Therein comes into play the authority factor. Authority and tradition. Generally speaking we don't respect stupidity in society unless it is of religious nature and even then only if it conforms to established religious patterns, hence my example of Poseidon.
I miss Western Europe, no one cares about religion and they just go about their lives; much better.
