Objection 1. It would seem that the beatified angels can advance in beatitude. For charity is the principle of merit. But there is perfect charity in the angels. Therefore the beatified angels can merit. Now, as merit increases, the reward of beatitude increases. Therefore the beatified angels can progress in beatitude.
Objection 2. Further, Augustine says (De Doctr. Christ. i) that "God makes use of us for our own gain, and for His own goodness. The same thing happens to the angels, whom He uses for spiritual ministrations"; since "they are all [Vulgate: 'Are they not all...?'] ministering spirits, sent to minister for them who shall receive the inheritance of salvation" (Hebrews 1:14). This would not be for their profit were they not to merit thereby, nor to advance to beatitude. It remains, then, that the beatified angels can merit, and can advance in beatitude.
Objection 3. Further, it argues imperfection for anyone not occupying the foremost place not to be able to advance. But the angels are not in the highest degree of beatitude. Therefore if unable to ascend higher, it would appear that there is imperfection and defect in them; which is not admissible.
On the contrary, Merit and progress belong to this present condition of life. But angels are not wayfarers travelling towards beatitude, they are already in possession of beatitude. Consequently the beatified angels can neither merit nor advance in beatitude.
I answer that, In every movement the mover's intention is centered upon one determined end, to which he intends to lead the movable subject; because intention looks to the end, to which infinite progress is repugnant. Now it is evident, since the rational creature cannot of its own power attain to its beatitude, which consists in the vision of God, as is clear from what has gone before (12, 4), that it needs to be moved by God towards its beatitude. Therefore there must be some one determined thing to which every rational creature is directed as to its last end.
Now this one determinate object cannot, in the vision of God, consist precisely in that which is seen; for the Supreme Truth is seen by all the blessed in various degrees: but it is on the part of the mode of vision, that diverse terms are fixed beforehand by the intention of Him Who directs towards the end. For it is impossible that as the rational creature is led on to the vision of the Supreme Essence, it should be led on in the same way to the supreme mode of vision, which is comprehension, for this belongs to God only; as is evident from what was said above (12, 7; 14, 3). But since infinite efficacy is required for comprehending God, while the creature's efficacy in beholding is only finite; and since every finite being is in infinite degrees removed from the infinite; it comes to pass that the rational creature understands God more or less clearly according to infinite degrees. And as beatitude consists in vision, so the degree of vision lies in a determinate mode of the vision.
Therefore every rational creature is so led by God to the end of its beatitude, that from God's predestination it is brought even to a determinate degree of beatitude. Consequently, when that degree is once secured, it cannot pass to a higher degree.
Reply to Objection 1. Merit belongs to a subject which is moving towards its end. Now the rational creature is moved towards its end, not merely passively, but also by working actively. If the end is within the power of the rational creature, then its action is said to procure the end; as man acquires knowledge by reflection: but if the end be beyond its power, and is looked for from another, then the action will be meritorious of such end. But what is already in the ultimate term is not said to be moved, but to have been moved. Consequently, to merit belongs to the imperfect charity of this life; whereas perfect charity does not merit but rather enjoys the reward. Even as in acquired habits, the operation preceding the habit is productive of the habit; but the operation from an acquired habit is both perfect and enjoyable. In the same way the act of perfect charity has no quality of merit, but belongs rather to the perfection of the reward.
Reply to Objection 2. A thing can be termed useful in two ways.
First of all, as being on the way to an end; and so the merit of beatitude is useful.
Secondly, as the part is useful for the whole; as the wall for a house. In this way the angelic ministerings are useful for the beatified angels, inasmuch as they are a part of their beatitude; for to pour out acquired perfection upon others is of the nature of what is perfect, considered as perfect.
Reply to Objection 3. Although a beatified angel is not absolutely in the highest degree of beatitude, yet, in his own regard he is in the highest degree, according to Divine predestination. Nevertheless the joy of the angels can be increased with regard to the salvation of such as are saved by their ministrations, according to Luke 15:10: "There is [Vulg.'shall be'] joy before the angels of God upon one sinner doing penance." Such joy belongs to their accidental reward, which can be increased unto judgment day. Hence some writers say that they can merit as to their accidental reward. But it is better to say that the Blessed can in no wise merit without being at the same time a wayfarer and a comprehensor; like Christ, Who alone was such. For the Blessed acquire such joy from the virtue of their beatitude, rather than merit it.
Source: Summa Theologica (New Advent)