1 Christ warned His followers against false prophets.
All things whatsoever our Lord and Saviour Jesus Christ, as Luke wrote, 'both has done and taught,' He effected after having appeared for our salvation; for He came, as John says, 'not to condemn the world, but that the world through Him might be saved.' And among the rest we have especially to admire this instance of His goodness, that He was not silent concerning those who should fight against us, but plainly told us beforehand, that, when those things should come to pass, we might straightway be found with minds established by His teaching. For He said, 'There shall arise false prophets and false Christs, and shall show great signs and wonders; insomuch that, if it were possible, the very elect shall be deceived. Behold, I have told you before.' Manifold indeed and beyond human conception are the instructions and gifts of grace which He has laid up in us; as the pattern of heavenly conversation, power against demons, the adoption of sons, and that exceeding great and singular grace, the knowledge of the Father and of the Word Himself, and the gift of the Holy Ghost. But the mind of man is prone to evil exceedingly; moreover, our adversary the devil, envying us the possession of such great blessings, goes about seeking to snatch away the seed of the word which is sown within us. Wherefore as if by His prophetic warnings He would seal up His instructions in our hearts as His own peculiar treasure, the Lord said, 'Take heed that no man deceive you: for many shall come in My name, saying, I am he; and the time draws near; and they shall deceive many: go ye not therefore after them.' This is a great gift which the Word has bestowed upon us, that we should not be deceived by appearances, but that, howsoever these things are concealed, we should all the more distinguish them by the grace of the Spirit. For whereas the inventor of wickedness and great spirit of evil, the devil, is utterly hateful, and as soon as he shows himself is rejected of all men—as a serpent, as a dragon, as a lion seeking whom he may seize upon and devour—therefore he conceals and covers what he really is, and craftily personates that Name which all men desire, so that deceiving by a false appearance, he may thenceforth fix fast in his own chains those whom he has led astray. And as if one that desired to kidnap the children of others during the absence of their parents, should personate their appearance, and so putting a cheat on the affections of the offspring, should carry them far away and destroy them; in like manner this evil and wily spirit the devil, having no confidence in himself, and knowing the love which men bear to the truth, personates its appearance, and so spreads his own poison among those that follow after him.
2. Satan pretending to be holy, is detected by the Christian.
Thus he deceived Eve, not speaking his own, but artfully adopting the words of God, and perverting their meaning. Thus he suggested evil to the wife of Job, persuading her to feign affection for her husband, while he taught her to blaspheme God. Thus does the crafty spirit mock men by false displays, deluding and drawing each into his own pit of wickedness. When of old he deceived the first man Adam, thinking that through him he should have all men subject unto him, he exulted with great boldness and said, 'My hand has found as a nest the riches of the people; and as one gathers eggs that are left, have I gathered all the earth; and there is none that shall escape me or speak against me. ' But when the Lord came upon earth, and the enemy made trial of His human Economy, being unable to deceive the flesh which He had taken upon Him, from that time forth he, who promised himself the occupation of the whole world, is for His sake mocked even by children: that proud one is mocked as a sparrow. For now the infant child lays his hand upon the hole of the asp, and laughs at him that deceived Eve; and all that rightly believe in the Lord tread under foot him that said, 'I will ascend above the heights of the clouds: I will be like the Most High.' Thus he suffers and is dishonoured; and although he still ventures with shameless confidence to disguise himself, yet now, wretched spirit, he is detected the rather by them that bear the Sign on their foreheads; yea, more, he is rejected of them, and is humbled, and put to shame. For even if, now that he is a creeping serpent, he shall transform himself into an angel of light, yet his deception will not profit him; for we have been taught that 'though an angel from heaven preach unto us any other gospel than that we have received, he is anathema.'
3. And although, again, he conceal his natural falsehood, and pretend to speak truth with his lips; yet are we 'not ignorant of his devices,' but are able to answer him in the words spoken by the Spirit against him; 'But unto the ungodly, said God, why do you preach My laws?' and, 'Praise is not seemly in the mouth of a sinner. ' For even though he speak the truth, the deceiver is not worthy of credit. And whereas Scripture showed this, when relating his wicked artifices against Eve in Paradise, so the Lord also reproved him—first in the mount, when He laid open 'the folds of his breast-plate,' and showed who the crafty spirit was, and proved that it was not one of the saints, but Satan that was tempting Him. For He said, 'Get behind Me Satan; for it is written, You shall worship the Lord your God, and Him only shall you serve.' And again, when He put a curb in the mouths of the demons that cried after Him from the tombs. For although what they said was true, and they lied not then, saying, 'You are the Son of God,' and 'the Holy One of God;' yet He would not that the truth should proceed from an unclean mouth, and especially from such as them, lest under pretence thereof they should mingle with it their own malicious devices, and sow these also while men slept. Therefore He suffered them not to speak such words, neither would He have us to suffer such, but has charged us by His own mouth, saying, 'Beware of false prophets, which come to you in sheeps' clothing, but inwardly they are ravening wolves;' and by the mouth of His Holy Apostles, 'Believe not every spirit.' Such is the method of our adversary's operations; and of the like nature are all these inventions of heresies, each of which has for the father of its own device the devil, who changed and became a murderer and a liar from the beginning. But being ashamed to profess his hateful name, they usurp the glorious Name of our Saviour 'which is above every name,' and deck themselves out in the language of Scripture, speaking indeed the words, but stealing away the true meaning thereof; and so disguising by some artifice their false inventions, they also become the murderers of those whom they have led astray.
4. It profits not to receive part of Scripture, and reject part.
Source: Ad Episcopus Aegypti et Libyae (New Advent)