The Catholic Church, for its part, fully sustained the efforts of the political authorities to advance the temporal welfare of all the people. In the spiritual realm the Church took very seriously its mission to teach, govern, and sanctify the faithful. In her language the Church is the union of all the faithful using common means – the Sacraments – to attain a common end – heaven – under a common authority – the Pope. Since, then, it had the duty of proclaiming the truths of Christ’s doctrine, by the same token it had the corresponding responsibility of exposing heretical ideas and practices. The crime of heresy was defined as a deliberate denial of some article of revealed truth of the Catholic faith, and a public and obstinate persistence in that error.
Albert Shannon, The Medieval Inquisition (1991)