From The Collected Works of St. Teresa of Avila, Volume 1, Chapter 31, parts: 7-11 :
7. It is certain that what I shall now tell happened to me. (And there are many witnesses to this, especially the one who is now my confessor since he saw it written in a letter; without my telling him who the person was to whom the letter belonged, he knew very well who it was.)
10. May what was said be of help that the true servant of God might pay no attention to the scarecrows the devils set up in order to cause fear. We should know that each time we pay no attention to them they are weakened and the soul gains much more mastery. Some great benefit always remains, which I won't go into so as not to enlarge. I shall only mention what happened to me on the night of All Souls: while I was in the oratory after having recited a nocturn and while saying some very devotional prayers that come at the end, a devil appeared on the book so that I couldn't finish the prayer. I blessed myself, and he went away. When I began again to recite the prayers, he returned. I believe it was three times I began, and until I threw holy water at him I couldn't finish. I saw that some souls left purgatory at that instant; little must have been lacking to their freedom, and I wondered if he had aimed at preventing this.
A few times I've seen him in physical form, but many times with no physical form-as for instance in the vision mentioned above in which without seeing any form one knows he is there.
At other times I saw a large multitude of devils around me, and it seemed that a great brightness encircled me, and this prevented them from reaching me. I understood that God was watching over me so that they could not get to me in order to make me offend Him. From what I sometimes saw in myself, I understood that it was a true vision. The fact is that now I have understood so well the little bit of power he has provided I'm not against God, that I have almost no fear . The powers of devils are nothing if these devils do not find souls cowardly and surrendered to them: it is with such souls that they show their power. Sometime, in the temptations I already mentioned, it seemed to me that all the vanities and weaknesses of the past were again awakening within me; I had really to commend myself to God. At once the torment came of thinking that since those thoughts arose in me the favors I experienced must all be from the devil. It seemed to me that there shouldn't have been even the first stirrings of a bad thought in one who was receiving so many favors from the Lord. But then my confessor put me at peace.
The Collected Works of St. Teresa of Avila, Volume 1, Chapter 31, part 4.