She believed in God, as do most Unitarians in one form or another, but didn't believe in the Devil. As much as she longed to understand the Devil and Hell, I longed to understand how she could believe in God, but not the rest of the set-up. It's like believing day, but not night.
"I am who am," God has clearly stated in the bible to the Israelites. The devil has somehow managed to convince people, "I am NOT." The angels were all given God's grace, but the fallen angels, through pride, would not accept God's grace.
According to St. Thomas (Summa Theologica, First Part, Question 63, Article 2):
Consequently the first sin of the angel can be none other than pride.
Yet, as a consequence, it was possible for envy also to be in them, since for the appetite to tend to the desire of something involves on its part resistance to anything contrary. Now the envious man repines over the good possessed by another, inasmuch as he deems his neighbor's good to be a hindrance to his own. But another's good could not be deemed a hindrance to the good coveted by the wicked angel, except inasmuch as he coveted a singular excellence, which would cease to be singular because of the excellence of some other. So, after the sin of pride, there followed the evil of envy in the sinning angel, whereby he grieved over man's good, and also over the Divine excellence, according as against the devil's will God makes use of man for the Divine glory.
I have heard it explained several times, all pretty much the same idea, that the test the angels underwent was they were shown that Christ would be born of a woman, a mere mortal, and they refused to accept that God would condescend so low and that He even expected the angels, who are of intellects so beyond ours, to bow down to this child.
One can only imagine the intellect of Lucifer, said to be the most brilliant of all the angels, and his complete disgust at the idea.
St. Thomas uses St. Augustine's idea of the angels having "morning" and "evening" knowledge. (Summa Theologica 1.58.6)
The expression "morning" and "evening" knowledge was devised by Augustine; who interprets the six days wherein God made all things, not as ordinary days measured by the solar circuit, since the sun was only made on the fourth day, but as one day, namely, the day of angelic knowledge as directed to six classes of things. As in the ordinary day, morning is the beginning, and evening the close of day, so, their knowledge of the primordial being of things is called morning knowledge; and this is according as things exist in the Word. But their knowledge of the very being of the thing created, as it stands in its own nature, is termed evening knowledge; because the being of things flows from the Word, as from a kind of primordial principle; and this flow is terminated in the being which they have in themselves.
St. Thomas summarizes this idea much better than I could:
The morning and evening knowledge belong to the day, that is, to the enlightened angels, who are quite apart from the darkness, that is, from the evil spirits. The good angels, while knowing the creature, do not adhere to it, for that would be to turn to darkness and to night; but they refer this back to the praise of God, in Whom, as in their principle, they know all things. Consequently after "evening" there is no night, but "morning"; so that morning is the end of the preceding day, and the beginning of the following, in so far as the angels refer to God's praise their knowledge of the preceding work. Noonday is comprised under the name of day, as the middle between the two extremes. Or else the noon can be referred to their knowledge of God Himself, Who has neither beginning nor end.
This, to me is a very interesting way to understand the knowledge of the angels, which had always been described to me as "infused knowledge." Infused to me always meant immediate or innate. This description seems to imply that angels can grow or move in understanding, moving from the knowledge of themselves (evening knowledge) to knowledge of God (morning knowledge).
Reading further in the Summa, St. Thomas discusses the will and free choice on the angels. I had learned that the angels were put to "the test," and because their intellect is so beyond ours, they understood all the ramifications, all the consequences, and had all the knowledge to make a completely informed choice, and that because of this, their choosing to serve God, or not, was irreversible and forever. No chance for regret, no repentence, no prodigal sons, no pity parties.
From the Summa (1.59.3)
Some things there are which act, not from any previous judgment, but, as it were, moved and made to act by others; just as the arrow is directed to the target by the archer. Others act from some kind of judgment; but not from free-will, such as irrational animals; for the sheep flies from the wolf by a kind of judgment whereby it esteems it to be hurtful to itself: such a judgment is not a free one, but implanted by nature. Only an agent endowed with an intellect can act with a judgment which is free, in so far as it apprehends the common note of goodness; from which it can judge this or the other thing to be good. Consequently, wherever there is intellect, there is free-will. It is therefore manifest that just as there is intellect, so is there free-will in the angels, and in a higher degree of perfection than in man.
I like St. Thomas' explanation on the knowledge of the angels since it helps me understand a bit better, like St. Patrick's analogy of using the clover to represent the Trinity. The devil lives in a perpetual night of knowledge, never choosing to look at the morning -- which is God. Almost akin to our faculties being clouded because of sin.