“But while the preaching of the divine Word should never be interrupted in the Church, surely in these, our days, it becomes necessary to labour with more than ordinary zeal and piety to nourish and strengthen the faithful with sound and wholesome doctrine, as with the food of life. For false prophets have gone forth into the world, to corrupt the minds of the faithful with various and strange doctrines, of whom the Lord has said: I did not send prophets, yet they ran; I spoke not to them, yet they prophesied.
In this work, to such extremes has their impiety, practiced in all the arts of Satan, been carried, that it would seem almost impossible to confine it within any bounds; and did we not rely on the splendid promises of the Saviour, who declared that He had built His Church on so solid a foundation that the gates of hell shall not prevail against it, we should have good reason to fear lest, beset on every side by such a host of enemies and assailed and attacked by so many machinations, it would, in these days, fall to the ground.
For – to say nothing of those illustrious states which heretofore professed, in piety and holiness, the true Catholic Faith transmitted to them by their ancestors, but are now gone astray, wandering from the paths of truth and openly declaring that their best claims to piety are founded on a total abandonment of the faith of their fathers – there is no region, however remote, no place, however securely guarded, no corner of Christendom, into which this pestilence has not sought secretly to insinuate itself.
For those who intended to corrupt the minds of the faithful, knowing that they could not hold immediate personal intercourse with all, and thus pour into their ears their poisoned doctrines, adopted another plan which enabled them to disseminate error and impiety more easily and extensively. Besides those voluminous works by which they sought the subversion of the Catholic faith – to guard against which (volumes) required perhaps little labour or circumspection, since their contents were clearly heretical – they also composed innumerable smaller books, which, veiling their errors under the semblance of piety, deceived with incredible facility the unsuspecting minds of simple folk.”
Catechism of the Council of Trent
In our day individual Catholics need to ensure that they are well instructed as the people who intend to corrupt the faith are generally Catholic Churchmen.