Against False Teaching

Quia non omnium est fides, et insidiosus ille tentator nullis magis corda hominum gaudet sauciare vulneribus, quam ut incautas mentes his inficiat erroribus, qui sint evangelicae veritatis inimici, magna sancti Spiritus eruditione nitendum est ut diabolicis falsitibus nequeat intelligentia Christiana violari. Contra quod periculum praecipue convenit vigilare Ecclesiarum rectores, et quadam verisimilitudune colorata mendacia ab animis simplicis plebis avertere. Angusta enim et ardua via est quae ducit ad vitam. Nec tantum laquerorum in actionum observantia, quantum inter discrimina sensuum latet, dum vis sententiarum brevissima adjectione aut commutatione corrumpitur et confessio quae operabatur salutem, subtili nonnumquam transitu vergit in mortem. Cum autem dicat Apostolus: Oportet haereses esse ut probati manifesti fiant in vobis, ad totius Ecclesiae tendit profectum, quoties contrariorum sensuum se prodit impietas, ne quae sunt noxia sint occulta et aliorum laedant incolumitatem, quae revocari non poterunt ad salutem. Unde sibi impudent ruinas et obcaecationes suas, qui imprudenti pertinacia jacere in reatu suo, quam oblatum remedium sumere, maluerunt. Sanctus Leo Magnus, ex Epistola CXXIX Proterio Episcopo Alexandrino

Because all do not have faith and the crafty Tempter delights in nothing more than wounding the hearts of men as when he can poison their unwary minds with errors that are inimical to the true gospel, we must strive by the mighty teaching of the Holy Spirit to prevent Christian knowledge from being perverted by the devil's falsehoods. And against this danger it is the especial duty of the rulers of the churches to guard and to avert from the minds of simple folk lies which are coloured by a certain show of truth.  Narrow and difficult is the way which leads to life. And they seek to entrap men not so much by watching their actions as by nice distinctions of meaning, corrupting the force of sentences by some very slight addition or alteration, whereby sometimes a statement, which was made for salvation, by a subtle change is turned to destruction. But since the Apostle says, 'There must be heresies that they who are approved may be made manifest among you,'1 it tends to the progress of the whole Church, that, whenever wickedness reveals itself in setting forth wrong opinions, the things which are harmful are not concealed, and that what will inevitably end in ruin may not injure the innocence of others. Thus they must attribute to themselves their ruin and blindness, they who with reckless obstinacy prefer to glory in their shame than to receive the offered remedy. Pope Leo the Great, from Letter 129 to Protetius Bishop of Alexandria 1 1 Cor 11.19