I think everyone understands how one's view of the world colors one's perceptions of it. It is a variation of the condition commonly referred to as "conflict of interest", where our most sincerely held beliefs often tend to coincide with our own personal financial requirements. Unfortunately, most of us feel this is a condition that affects others, and never oneself. An ideological position, however carefully thought out and supported by logic, once firmly entrenched can become extremely difficult to dislodge by the acquisition of new facts or by the rebuttal of previously held notions. This is not a question of intellectual dishonesty or even of stupidity or ignorance, it is simply the way the human brain is wired, and it affects all of us to some extent and some of us in an extreme and crippling way. Every mind exhibits some ideological inertia, and perhaps it is even of some value, keeping us from abandoning valuable hypotheses for dealing with reality because of occasional flimsy evidence to the contrary.

We are all familiar with individuals who insist on maintaining beliefs even after evidence to the contrary has become so convincing and detailed that their fellows simply cannot fathom why the obviously incorrect opinions continue to be supported. Unfortunately, intelligence sometimes gets in the way rather than helps here; there are those of us who exhibit bright and agile minds but devote this intellectual power to justifying the unjustifiable. Every fact and observation is twisted by logic and inference to maintain the incorrect ideology, sometimes very cleverly and consistently. People view the world through lenses, through filters. In ideological matters such as religion or politics the emotional attachment for the incorrect view is so powerful that its rejection becomes tantamount to personal defeat and humiliation. Even if a modification of one's views is not equivalent to rejecting a core belief (for example, accepting the truth of Darwinian natural selection is NOT a rejection of Christian religious values), if it is perceived this way, and if the two are linked, then it becomes necessary to defend every attack on one's position, no matter how limited or specific, as an attack on one's entire world view.

We all know people like this, they feel obliged to question even the slightest deviation from their orthodoxy, not just in the main philosophical axioms of their belief, but even in the tiniest detail that might remotely threaten any part of their ideology. They will interrupt the conversations of others, attack perfect strangers, employ sarcasm and ridicule to humiliate and discredit their opponents, or even those they suspect might become their opponents. It's like they are constantly on patrol, looking for the slightest deviation in others from their own orthodoxy so they can pounce on it and crush the offender. This action is often in complete contrast to their normal behavior when discussing other subjects. A courteous and rational conversationalist will be transformed unexpectedly into a toxic, venemous nag once a key phrase or concept appears and is assessed as a threat. Even if a declaration of neutrality or moderation is made by the opponent, or a hint of compromise is offered, it is often rejected as a form of conversational Trojan Horse. Eventually, the opponent is attacked personally, his character and virtue questioned directly, anything to demonstrate that he is morally or intellectually unfit to argue the other side, or better yet, that his alleged sympathy for the other side has made him unfit.

We are all familiar with examples of this phenomenon, perhaps the most obvious historical case is the rejection by leftists of the rapidly accumulating evidence of the enormous crimes of the Stalinist regime in the Soviet Union. A very similar condition today, one I feel I have some personal experience with, is the support of the Castro regime in Cuba, sometimes even by people who do not consider themselves leftists. The irony that many of the victims of these dictatorships were Communists themselves seem to be lost on these people. In our own country, the continued support by conservatives of the Nixon Administration, even after its crimes had been completely exposed, also comes to mind. I'm sure you can come up with further, more recent examples of your own.

You cannot reason with people like this. Just let them go.