Saint Ignatius Loyola (1491-1556), Founder of the Jesuits
Spiritual Exercises: rules for distinguishing spiritual influences (trans. Thomas Corbishley, SJ)
The characteristic effect produced by God and His angels in their spiritual operations is a genuine lightness of heart and spiritual joy, eliminating all the disturbing sadness engendered by the enemy, whilst his characteristic activity is to resist such lightness of heart and spiritual comfort, alleging specious reasons, subtle suggestions and sophistries without end. Spiritual comfort with no previous occasion giving rise to it comes from our Lord God alone. It is the Creator's prerogative to come into and leave the soul, to move it with inspirations of love for His Divine Majesty. "With no previous occasion" means without any preceding awareness or knowledge of anything which might induce such comfort in the soul...
It is typical of the evil spirit to transform himself into an angel of light, to go in by the devout soul's way but to come out his own way; I mean he introduces sound and pious thoughts, suited to the piety of that soul; but then, little by little, he tries to achieve his own purposes, by dragging the soul down to his secret designs and corrupt purposes.
We should pay great attention to the entire train of thought. If beginning, middle and end are wholly sound, tending to what is completely innocent, this is a sign of the good angel; but the train of thought suggested sometimes leads to something that is bad or at least distracting, or less good than what the soul had originally proposed to do; sometimes it undermines our strength of mind or disturbs us by destroying our peace and tranquillity of mind and the unperturbed condition already obtaining: these are clear signs that the thoughts come from the evil spirit, the enemy of our progress and everlasting salvation... When souls are advancing from good to better, the touch of the good angel is soft, light and gentle, like a drop of water making its way into a sponge. The touch of the evil angel is rough, accompanied by noise and disturbance, like a drop of water falling on stone.