When arguing with a believer, at what point does the continued attempt to convince him of his error begin to say more about the interrogator than his irrational opponent?

If we are going to defend the Hermeneutics of suspicion wouldn't it make sense to begin with ourselves?

Looking through Simon Blackburn's Oxford Dictionary of Philosophy I found a quote from Coleridge:

He who begins by loving Christianity better than Truth, will proceed by loving his own sect or Church better than Christianity, and in the end loving himself better than all.

One can design arguments that make black, white, and the reverse; scientists can become deluded, and policemen, contrary to what are often are their own assumptions, are not Law. I've always thought that Dworkin attempts to describe the process by which self-reflexive doubt is institutionalized in legal decision making, and doubt is not a quantitative understanding of the world. Rescue Marx and Freud from science and you rescue them from faith.