Saint Symeon Metaphrastis. "Paraphrase of the Homilies of Saint Makarios of Egypt". VI The Freedom of the Intellect. From Philokalia, Vol. 3.
122. One must watch very carefully and in every direction for the enemy's trickery, guile and malice. Speaking through St Paul, the Holy Spirit says: 'I became all things to all men so as to save everyone' (cf. 1 Cor. 9:22); and in the same way the enemy tries to become all things so as to bring everyone to destruction. He pretends to pray with those who pray, so as to cheat them into self-conceit by making them think they have attained the state of prayer. He fasts with those who fast, with the purpose again of filling them with self-conceit because they have succeeded in fasting. With those who understand the Scriptures he tries to do the same, hoping to lead them astray by making them claim to possess spiritual knowledge. To those who have been granted a vision of the light, he pretends to offer the same kind of vision, transforming himself into 'an angel of light' (2 Cor. 11:14), so that through this simulation of the true light he may seduce them to him. In short, he uses every kind of deceit and adapts himself to every kind of appearance, so that by assuming the likeness of what is good he becomes a plausible agent of destruction. 'We destroy evil thoughts and all the self-esteem that exalts itself against the knowledge of God', says St Paul (2 Cor. 10:5). You see to what limits the impostor carries his defiance, wanting to cast down even those who have already attained a divine knowledge of the truth. Hence we must guard the heart with all diligence and beseech God for much understanding, so that He enables us to discern the devil's wiliness, to cultivate and train the intellect in understanding, to attend continually to our thoughts, and to conform ourselves to His will. There is no work greater than this. 'Praise and magnificence are His works', as the psalmist writes (Ps. 111:3. LXX).