Objection 1. It would seem that an angel can be in several places at once. For an angel is not less endowed with power than the soul. But the soul is in several places at once, for it is entirely in every part of the body, as Augustine says (De Trin. vi). Therefore an angel can be in several places at once.
Objection 2. Further, an angel is in the body which he assumes; and, since the body which he assumes is continuous, it would appear that he is in every part thereof. But according to the various parts there are various places. Therefore the angel is at one time in various places.
Objection 3. Further, Damascene says (De Fide Orth. ii) that "where the angel operates, there he is." But occasionally he operates in several places at one time, as is evident from the angel destroying Sodom (Genesis 19:25). Therefore an angel can be in several places at the one time.
On the contrary, Damascene says (De Fide Orth. ii) that "while the angels are in heaven, they are not on earth."
I answer that, An angel's power and nature are finite, whereas the Divine power and essence, which is the universal cause of all things, is infinite: consequently God through His power touches all things, and is not merely present in some places, but is everywhere. Now since the angel's power is finite, it does not extend to all things, but to one determined thing. For whatever is compared with one power must be compared therewith as one determined thing. Consequently since all being is compared as one thing to God's universal power, so is one particular being compared as one with the angelic power. Hence, since the angel is in a place by the application of his power to the place, it follows that he is not everywhere, nor in several places, but in only one place.
Some, however, have been deceived in this matter. For some who were unable to go beyond the reach of their imaginations supposed the indivisibility of the angel to be like that of a point; consequently they thought that an angel could be only in a place which is a point. But they were manifestly deceived, because a point is something indivisible, yet having its situation; whereas the angel is indivisible, and beyond the genus of quantity and situation. Consequently there is no occasion for determining in his regard one indivisible place as to situation: any place which is either divisible or indivisible, great or small suffices, according as to his own free-will he applies his power to a great or to a small body. So the entire body to which he is applied by his power, corresponds as one place to him.
Neither, if any angel moves the heavens, is it necessary for him to be everywhere. First of all, because his power is applied only to what is first moved by him. Now there is one part of the heavens in which there is movement first of all, namely, the part to the east: hence the Philosopher (Phys. vii, text 84) attributes the power of the heavenly mover to the part which is in the east. Secondly, because philosophers do not hold that one separate substance moves all the spheres immediately. Hence it need not be everywhere.
So, then, it is evident that to be in a place appertains quite differently to a body, to an angel, and to God. For a body is in a place in a circumscribed fashion, since it is measured by the place. An angel, however, is not there in a circumscribed fashion, since he is not measured by the place, but definitively, because he is in a place in such a manner that he is not in another. But God is neither circumscriptively nor definitively there, because He is everywhere.
From this we can easily gather an answer to the objections: because the entire subject to which the angelic power is immediately applied, is reputed as one place, even though it be continuous.
Source: Summa Theologica (New Advent)