Excerpt from The Life of St. Antony:

The demons, therefore, if they see all Christians, and monks especially, labouring cheerfully and advancing, first make an attack by temptation and place hindrances to hamper our way, to wit, evil thoughts. But we need not fear their suggestions, for by prayer, fasting, and faith in the Lord their attack immediately fails. But even when it does they cease not, but knavishly by subtlety come on again. For when they cannot deceive the heart openly with foul pleasures they approach in different guise, and thenceforth shaping displays they attempt to strike fear, changing their shapes, taking the forms of women, wild beasts, creeping things, gigantic bodies, and troops of soldiers. But not even then need ye fear their deceitful displays. For they are nothing and quickly disappear, especially if a man fortify himself beforehand with faith and the sign of the cross. Yet are they bold and very shameless, for if thus they are worsted they make an onslaught in another manner, and pretend to prophesy and foretell the future, and to shew themselves of a height reaching to the roof and of great breadth; that they may stealthily catch by such displays those who could not be deceived by their arguments. If here also they find the soul strengthened by faith and a hopeful mind, then they bring their leader to their aid.