From St. Teresa of Avila.
4. I can assure you, my daughters, 5 that I have known people far advanced in the spiritual life who had reached this state of prayer yet whom the devil reclaimed by his subtlety and wiles: as I have often said, all hell leagues together against such souls because the loss of one of them entails the perdition of many more, as Satan is well aware. If we considered how many men God draws to Himself by means of one, we should praise Him fervently. Think of the multitudes converted by the martyrs or by one young maiden like St. Ursula! Again, of how many victims the evil one was deprived by St. Dominic, St. Francis, and other founders of religious orders. How many more he loses, even now, through Father Ignatius [Loyola], who founded the Company [of Jesus]! As we read their lives, we learn that they received such graces from God. How was this great good done except by their efforts not to forfeit, through any fault of theirs, these divine espousals? Oh, my daughters, how willing our Lord is to grant us the same graces! In fact, there is even more urgent need now for persons to prepare for such favours, since there are fewer who care for His honour. We love ourselves too much and are too prudent to give up any of our rights. What a deception! May God in His mercy give us light, lest we sink into such darkness.
5. You may question or be in doubt on two points. Firstly: if the soul is entirely united with the will of God, as I have stated, how can it be deceived, since it ever seeks to follow His pleasure? Secondly, how can the devil enter and work such havoc as to destroy your soul while you are so utterly withdrawn from the world and constantly frequent the Sacraments? 6 At the same time you enjoy the society of angels (as we might call them) and by the mercy of God you desire nothing but to serve and please Him in all things? 7 It is not surprising that people in the world should run such risks. I admit you have the right to say this, for God has shown us signal mercy; but, as I said above, knowing as I do that Judas was amongst the Apostles and that he held constant intercourse with God Himself, to Whose words he listened, I learn that the state of religion does not make us safe.
6. To your first question I reply that doubtless if such a soul is always faithful to the will of God, it cannot be lost; the evil one, however, comes with his keen subtlety and, under the pretext of good, leads it astray in some trivial matter and causes it to commit small defects which he makes it believe are harmless. Thus, little by little, the reason is obscured and the will is weakened while the devil fosters his victim's self-love, until, by degrees, he succeeds in withdrawing it from union with the will of God and makes it follow its own will.
7. The answer to your first inquiry will serve for the second. No enclosure can be too strict for Satan to enter nor any desert too remote for him to visit. Besides, God may permit him to tempt the soul to prove its virtue; for as He intends it to enlighten others, it is better for it to fail in the beginning than when it might do them great harm.
8. We must beg God constantly in our prayers to uphold us by His hand; we should keep ever in our minds the truth that if He leaves us, most certainly we shall fall at once into the abyss, for we must never be so foolish as to trust in ourselves. After this I think the greatest safeguard is to be very careful and to watch how we advance in virtue; we must notice whether we are making progress or falling back in it, especially as regards the love of our neighbour, the desire to be thought the least of all and how we perform our ordinary, everyday duties. If we attend to this and beg Our Lord to enlighten us, we shall at once perceive our gain or loss. - Interior Castle