Lord warns us to watch and pray. It is indispensable, then, that you should use the utmost vigilance, in order to prevent the enemy from robbing you of so great a treasure as peace and quietness of mind; for with all his might he tries to deprive you of this repose, and to make you live in continual unrest, knowing that such a state is fraught with danger and injury to your soul: for if a soul is at peace, all things are done with ease, and great things are done well; hence it willingly perseveres and surmounts all opposition with ease. And on the other hand, if it is disturbed and unquiet, it does but little, and that little very imperfectly, and soon becomes weary, and, in fact, lives in a fruitless martyrdom. If you wish, then, to come off victorious, and to foil the enemy, when he attempts to destroy your labours, there is nothing about which you must be more on your guard than not to let your soul become disturbed, nor to consent even for a moment to any temptation to disquietude.
And that you may know the better how to guard yourself against the wiles of the enemy, make it a certain rule in this case, that every thought which discourages you and lessens your love and confidence towards God, is a messenger from hell, and therefore to be driven away, and banished from your presence without an audience. For the office of the Holy Spirit is none
