Objection 1. It would seem that the angel had no need of grace in order to turn to God. For, we have no need of grace for what we can accomplish naturally. But the angel naturally turns to God: because he loves God naturally, as is clear from what has been said (60, 5). Therefore an angel did not need grace in order to turn to God.
Objection 2. Further, seemingly we need help only for difficult tasks. Now it was not a difficult task for the angel to turn to God; because there was no obstacle in him to such turning. Therefore the angel had no need of grace in order to turn to God.
Objection 3. Further, to turn oneself to God is to dispose oneself for grace; hence it is said (Zechariah 1:3): "Turn ye to Me, and I will turn to you." But we do not stand in need of grace in order to prepare ourselves for grace: for thus we should go on to infinity. Therefore the angel did not need grace to turn to God.
On the contrary, It was by turning to God that the angel reached to beatitude. If, then, he had needed no grace in order to turn to God, it would follow that he did not require grace in order to possess everlasting life. But this is contrary to the saying of the Apostle (Romans 6:23): "The grace of God is life everlasting."
I answer that, The angels stood in need of grace in order to turn to God, as the object of beatitude. For, as was observed above (Question 60, Article 2) the natural movement of the will is the principle of all things that we will. But the will's natural inclination is directed towards what is in keeping with its nature. Therefore, if there is anything which is above nature, the will cannot be inclined towards it, unless helped by some other supernatural principle. Thus it is clear that fire has a natural tendency to give forth heat, and to generate fire; whereas to generate flesh is beyond the natural power of fire; consequently, fire has no tendency thereto, except in so far as it is moved instrumentally by the nutritive soul.
Now it was shown above (12, 4,5), when we were treating of God's knowledge, that to see God in His essence, wherein the ultimate beatitude of the rational creature consists, is beyond the nature of every created intellect. Consequently no rational creature can have the movement of the will directed towards such beatitude, except it be moved thereto by a supernatural agent. This is what we call the help of grace. Therefore it must be said that an angel could not of his own will be turned to such beatitude, except by the help of grace.
Reply to Objection 1. The angel loves God naturally, so far as God is the author of his natural being. But here we are speaking of turning to God, so far as God bestows beatitude by the vision of His essence.
Reply to Objection 2. A thing is "difficult" which is beyond a power; and this happens in two ways.
First of all, because it is beyond the natural capacity of the power. Thus, if it can be attained by some help, it is said to be "difficult"; but if it can in no way be attained, then it is "impossible"; thus it is impossible for a man to fly. In another way a thing may be beyond the power, not according to the natural order of such power, but owing to some intervening hindrance; as to mount upwards is not contrary to the natural order of the motive power of the soul; because the soul, considered in itself, can be moved in any direction; but is hindered from so doing by the weight of the body; consequently it is difficult for a man to mount upwards. To be turned to his ultimate beatitude is difficult for man, both because it is beyond his nature, and because he has a hindrance from the corruption of the body and infection of sin. But it is difficult for an angel, only because it is supernatural.
Reply to Objection 3. Every movement of the will towards God can be termed a conversion to God. And so there is a threefold turning to God. The first is by the perfect love of God; this belongs to the creature enjoying the possession of God; and for such conversion, consummae grace is required. The next turning to God is that which merits beatitude; and for this there is required habitual grace, which is the principle of merit. The third conversion is that whereby a man disposes himself so that he may have grace; for this no habitual grace is required; but the operation of God, Who draws the soul towards Himself, according to Lamentations 5:21: "Convert us, O Lord, to Thee, and we shall be converted." Hence it is clear that there is no need to go on to infinity.
Source: Summa Theologica (New Advent)