The extirpation of heresy was given as the reason, but survival and relief from taxation or more direct expropriation was the more fundamental motivation. Support for the assertion of clerical preeminence is gained from the Synoptic Gospels, the Pauline Corpus and the Torah. In addition, a lengthy and sometimes successful track record can be adduced from the regula fidei, the Tradition, especially dating to Athanasius, Cyprian and Augustine. As strong as the tradition of clerical preeminence is, the assertion it rests on is untenable. A succession of prelates was obliged to straddle palfrey to defend it, but the successes were always short-lived.
Clergy lack the police power, by innate nature.
Their disposition is not for it. So they cannot sustain an assertion of temporal responsibility. Canonical materials which support the assertion of clerical preeminence can be discounted, by several courses of argument, and demonstrations of the assertion from Tradition indicate more an incapacity than a puissance, Hildebrand notwithstanding. What does not stand is not standable. What God does not approve, man may not establish.