Objection 1. It would seem that the angel did not possess beatitude instantly after one act of merit. For it is more difficult for a man to do well than for an angel. But man is not rewarded at once after one act of merit. Therefore neither was the angel.
Objection 2. Further, an angel could act at once, and in an instant, from the very outset of his creation, for even natural bodies begin to be moved in the very instant of their creation; and if the movement of a body could be instantaneous, like operations of mind and will, it would have movement in the first instant of its generation. Consequently, if the angel merited beatitude by one act of his will, he merited it in the first instant of his creation; and so, if their beatitude was not retarded, then the angels were in beatitude in the first instant.
Objection 3. Further, there must be many intervals between things which are far apart. But the beatific state of the angels is very far remote from their natural condition: while merit comes midway between. Therefore the angel would have to pass through many stages of merit in order to reach beatitude.
On the contrary, Man's soul and an angel are ordained alike for beatitude: consequently equality with angels is promised to the saints. Now the soul separated from the body, if it has merit deserving beatitude, enters at once into beatitude, unless there be some obstacle. Therefore so does an angel. Now an angel instantly, in his first act of charity, had the merit of beatitude. Therefore, since there was no obstacle within him, he passed at once into beatitude by only one meritorious act.
I answer that, The angel was beatified instantly after the first act of charity, whereby he merited beatitude. The reason whereof is because grace perfects nature according to the manner of the nature; as every perfection is received in the subject capable of perfection, according to its mode. Now it is proper to the angelic nature to receive its natural perfection not by passing from one stage to another; but to have it at once naturally, as was shown above (1; 58, 3,4). But as the angel is of his nature inclined to natural perfection, so is he by merit inclined to glory. Hence instantly after merit the angel secured beatitude. Now the merit of beatitude in angel and man alike can be from merely one act; because man merits beatitude by every act informed by charity. Hence it remains that an angel was beatified straightway after one act of charity.
Reply to Objection 1. Man was not intended to secure his ultimate perfection at once, like the angel. Hence a longer way was assigned to man than to the angel for securing beatitude.
Reply to Objection 2. The angel is above the time of corporeal things; hence the various instants regarding the angels are not to be taken except as reckoning the succession of their acts. Now their act which merited beatitude could not be in them simultaneously with the act of beatitude, which is fruition; since the one belongs to imperfect grace, and the other to consummate grace. Consequently, it remains for different instants to be conceived, in one of which the angel merited beatitude, and in another was beatified.
Reply to Objection 3. It is of the nature of an angel instantly to attain the perfection unto which he is ordained. Consequently, only one meritorious act is required; which act can so far be called an interval as through it the angel is brought to beatitude.
Source: Summa Theologica (New Advent)