Objection 1. It would seem that the angel did not merit his beatitude. For merit arises from the difficulty of the meritorious act. But the angel experienced no difficulty in acting rightly. Therefore righteous action was not meritorious for him.
Objection 2. Further, we do not merit by merely natural operations. But it was quite natural for the angel to turn to God. Therefore he did not thereby merit beatitude.
Objection 3. Further, if a beatified angel merited his beatitude, he did so either before he had it, or else afterwards. But it was not before; because, in the opinion of many, he had no grace before whereby to merit it. Nor did he merit it afterwards, because thus he would be meriting it now; which is clearly false, because in that case a lower angel could by meriting rise up to the rank of a higher, and the distinct degrees of grace would not be permanent; which is not admissible. Consequently the angel did not merit his beatitude.
On the contrary, It is stated (Apocalypse 21:17) that the "measure of the angel" in that heavenly Jerusalem is "the measure of a man." Therefore the same is the case with the angel.
I answer that, Perfect beatitude is natural only to God, because existence and beatitude are one and the same thing in Him. Beatitude, however, is not of the nature of the creature, but is its end. Now everything attains its last end by its operation. Such operation leading to the end is either productive of the end, when such end is not beyond the power of the agent working for the end, as the healing art is productive of health; or else it is deserving of the end, when such end is beyond the capacity of the agent striving to attain it; wherefore it is looked for from another's bestowing. Now it is evident from what has gone before (1,2; 12, 4,5), ultimate beatitude exceeds both the angelic and the human nature. It remains, then, that both man and angel merited their beatitude.
And if the angel was created in grace, without which there is no merit, there would be no difficulty in saying that he merited beatitude: as also, if one were to say that he had grace in any way before he had glory.
But if he had no grace before entering upon beatitude, it would then have to be said that he had beatitude without merit, even as we have grace. This, however, is quite foreign to the idea of beatitude; which conveys the notion of an end, and is the reward of virtue, as even the Philosopher says (Ethic. i, 9). Or else it will have to be said, as some others have maintained, that the angels merit beatitude by their present ministrations, while in beatitude. This is quite contrary, again, to the notion of merit: since merit conveys the idea of a means to an end; while what is already in its end cannot, properly speaking, be moved towards such end; and so no one merits to produce what he already enjoys. Or else it will have to be said that one and the same act of turning to God, so far as it comes of free-will, is meritorious; and so far as it attains the end, is the fruition of beatitude. Even this view will not stand, because free-will is not the sufficient cause of merit; and, consequently, an act cannot be meritorious as coming from free-will, except in so far as it is informed by grace; but it cannot at the same time be informed by imperfect grace, which is the principle of meriting, and by perfect grace, which is the principle of enjoying. Hence it does not appear to be possible for anyone to enjoy beatitude, and at the same time to merit it.
Consequently it is better to say that the angel had grace ere he was admitted to beatitude, and that by such grace he merited beatitude.
Reply to Objection 1. The angel's difficulty of working righteously does not come from any contrariety or hindrance of natural powers; but from the fact that the good work is beyond his natural capacity.
Reply to Objection 2. An angel did not merit beatitude by natural movement towards God; but by the movement of charity, which comes of grace.
The answer to the Third Objection is evident from what we have said.
Source: Summa Theologica (New Advent)