Why would anyone feel compelled to remind those who are minding their own business, as most likely do, that their beliefs are not rational? To me, beyond being rude, is condesencing and I suggest may also be seen as an attack. Why must you remind opeople of that?

Yes, it's rude. However, we are in an open discussion on an internet forum. No one here is accosting Christians with out-of-the-blue disparagement. I don't think you'll find many atheists who just start spouting off about how irrational people of faith are. If you are in a discussion about religion and claim that faith is rational, you should expect other people in that discussion to offer differing points of view. In other words, if you don't want people to point out that faith is irrational, don't get into conversations with them and claim that it is.