Objection 1. It would seem that the devil was wicked by the fault of his own will in the first instant of his creation. For it is said of the devil (John 8:44): "He was a murderer from the beginning."
Objection 2. Further, according to Augustine (Gen. ad lit. i, 15), the lack of form in the creature did not precede its formation in order of time, but merely in order of nature. Now according to him (Gen. ad lit. ii, 8), the "heaven," which is said to have been created in the beginning, signifies the angelic nature while as yet not fully formed: and when it is said that God said: "Be light made: and light was made," we are to understand the full formation of the angel by turning to the Word. Consequently, the nature of the angel was created, and light was made, in the one instant. But at the same moment that light was made, it was made distinct from "darkness," whereby the angels who sinned are denoted. Therefore in the first instant of their creation some of the angels were made blessed, and some sinned.
Objection 3. Further, sin is opposed to merit. But some intellectual nature can merit in the first instant of its creation; as the soul of Christ, or also the good angels. Therefore the demons likewise could sin in the first instant of their creation.
Objection 4. Further, the angelic nature is more powerful than the corporeal nature. But a corporeal thing begins to have its operation in the first instant of its creation; as fire begins to move upwards in the first instant it is produced. Therefore the angel could also have his operation in the first instant of his creation. Now this operation was either ordinate or inordinate. If ordinate, then, since he had grace, he thereby merited beatitude. But with the angels the reward follows immediately upon merit; as was said above (Question 62, Article 5). Consequently they would have become blessed at once; and so would never have sinned, which is false. It remains, then, that they sinned by inordinate action in their first instant.
On the contrary, It is written (Genesis 1:31): "God saw all the things that He had made, and they were very good." But among them were also the demons. Therefore the demons were at some time good.
I answer that, Some have maintained that the demons were wicked straightway in the first instant of their creation; not by their nature, but by the sin of their own will; because, as soon as he was made, the devil refused righteousness. To this opinion, as Augustine says (De Civ. Dei xi, 13), if anyone subscribes, he does not agree with those Manichean heretics who say that the devil's nature is evil of itself. Since this opinion, however, is in contradiction with the authority of Scripture--for it is said of the devil under the figure of the prince of Babylon (Isaiah 14:12): "How art thou fallen... O Lucifer, who didst rise in the morning!" and it is said to the devil in the person of the King of Tyre (Ezekiel 28:13): "Thou wast in the pleasures of the paradise of God," --consequently, this opinion was reasonably rejected by the masters as erroneous.
Hence others have said that the angels, in the first instant of their creation, could have sinned, but did not. Yet this view also is repudiated by some, because, when two operations follow one upon the other, it seems impossible for each operation to terminate in the one instant. Now it is clear that the angel's sin was an act subsequent to his creation. But the term of the creative act is the angel's very being, while the term of the sinful act is the being wicked. It seems, then, an impossibility for the angel to have been wicked in the first instant of his existence.
This argument, however, does not satisfy. For it holds good only in such movements as are measured by time, and take place successively; thus, if local movement follows a change, then the change and the local movement cannot be terminated in the same instant. But if the changes are instantaneous, then all at once and in the same instant there can be a term to the first and the second change; thus in the same instant in which the moon is lit up by the sun, the atmosphere is lit up by the moon. Now, it is manifest that creation is instantaneous; so also is the movement of free-will in the angels; for, as has been already stated, they have no occasion for comparison or discursive reasoning (58, 3). Consequently, there is nothing to hinder the term of creation and of free-will from existing in the same instant.
We must therefore reply that, on the contrary, it was impossible for the angel to sin in the first instant by an inordinate act of free-will. For although a thing can begin to act in the first instant of its existence, nevertheless, that operation which begins with the existence comes of the agent from which it drew its nature; just as upward movement in fire comes of its productive cause. Therefore, if there be anything which derives its nature from a defective cause, which can be the cause of a defective action, it can in the first instant of its existence have a defective operation; just as the leg, which is defective from birth, through a defect in the principle of generation, begins at once to limp. But the agent which brought the angels into existence, namely, God, cannot be the cause of sin. Consequently it cannot be said that the devil was wicked in the first instant of his creation.
Reply to Objection 1. As Augustine says (De Civ. Dei xi, 15), when it is stated that "the devil sins from the beginning," "he is not to be thought of as sinning from the beginning wherein he was created, but from the beginning of sin": that is to say, because he never went back from his sin.
Reply to Objection 2. That distinction of light and darkness, whereby the sins of the demons are understood by the term darkness, must be taken as according to God's foreknowledge. Hence Augustine says (De Civ. Dei xi, 15), that "He alone could discern light and darkness, Who also could foreknow, before they fell, those who would fall."
Reply to Objection 3. All that is in merit is from God; and consequently an angel could merit in the first instant of his creation. The same reason does not hold good of sin; as has been said.
Reply to Objection 4. God did not distinguish between the angels before the turning away of some of them, and the turning of others to Himself, as Augustine says (De Civ. Dei xi, 15). Therefore, as all were created in grace, all merited in their first instant. But some of them at once placed an impediment to their beatitude, thereby destroying their preceding merit; and consequently they were deprived of the beatitude which they had merited.
Source: Summa Theologica (New Advent)