Quote from : Dark Night of the Soul Book

The second cause whence these rebellions sometimes proceed is the devil, who, in order to disquiet and disturb the soul, at times when it is at prayer or is striving to pray, contrives to stir up these motions of impurity in its nature; and if the soul gives heed to any of these, they cause it great harm. For through fear of these not only do persons become lax in prayer—which is the aim of the devil when he begins to strive with them—but some give up prayer altogether, because they think that these things attack them more during that exercise than apart from it, which is true, since the devil attacks them then more than at other times, so that they may give up spiritual exercises.