“Beware of supposing that a teacher of religion is to be trusted, because although he holds some unsound views, he yet ‘teaches a great deal of truth.’ Such a teacher is precisely the man to do you harm: poison is always most dangerous when it is given in small doses and mixed with wholesome food. Beware of being taken in by the apparent earnestness of many of the teachers and upholders of false doctrine. Remember that zeal and sincerity and fervor are no proof whatever that a man is working for Christ, and ought to believed. Peter no doubt was in earnest when he bade our Lord spare Himself, and not go to the cross; yet our Lord said to him, ‘Get thee behind Me, Satan.’ Saul no doubt was in earnest when he went to and fro persecuting Christians; yet he did it ignorantly, and his zeal was not according to knowledge. The founders of the Spanish Inquisition no doubt were in earnest, and in burning God’s saints alive thought they were doing God service; yet they were actually persecuting Christ’s members and walking in the steps of Cain. – It is an awful fact that, ‘Satan himself is transformed into an angel of light.’ (2 Corinthians 11:14) Of all the delusions prevalent in these latter days, there is none greater than the common notion that ‘if a man is in earnest about his religion he must be a good man!’ Beware of being carried away by this delusion; beware of being led astray by ‘earnest minded men!’ Earnestness is in itself an excellent thing; but it must be earnestness in behalf of Christ and His whole truth, or else it is worth nothing at all.”

J.C. Ryle (1816-1900) “Warning to the Churches”                                                                   See more quotes at Monergism