By St. Paisios the Athonite
The devil can appear either as an angel or as a saint. The demon camouflaged as an angel or a saint will propagate disturbance (which is what he possesses), whereas a true Angel or Saint will always propagate a paradisiacal joy and a heavenly exuberance. The humble and clean person - even if inexperienced - can discern between an Angel that is from God and a demon that appears as an angel of light, because he is spiritually clean and more closely related to the Angel. The egotist and carnal person will however be deluded easily by the cunning devil. When the devil appears as an angel of light and the person forms a humble thought, the devil will vanish.