Objection 1. It seems that God is not infinite. For everything infinite is imperfect, as the Philosopher says; because it has parts and matter, as is said in Phys. iii. But God is most perfect; therefore He is not infinite.
Objection 2. Further, according to the Philosopher (Phys. i), finite and infinite belong to quantity. But there is no quantity in God, for He is not a body, as was shown above (Question 3, Article 1). Therefore it does not belong to Him to be infinite.
Objection 3. Further, what is here in such a way as not to be elsewhere, is finite according to place. Therefore that which is a thing in such a way as not to be another thing, is finite according to substance. But God is this, and not another; for He is not a stone or wood. Therefore God is not infinite in substance.
On the contrary, Damascene says (De Fide Orth. i, 4) that "God is infinite and eternal, and boundless."
I answer that, All the ancient philosophers attribute infinitude to the first principle, as is said (Phys. iii), and with reason; for they considered that things flow forth infinitely from the first principle. But because some erred concerning the nature of the first principle, as a consequence they erred also concerning its infinity; forasmuch as they asserted that matter was the first principle; consequently they attributed to the first principle a material infinity to the effect that some infinite body was the first principle of things.
We must consider therefore that a thing is called infinite because it is not finite. Now matter is in a way made finite by form, and the form by matter. Matter indeed is made finite by form, inasmuch as matter, before it receives its form, is in potentiality to many forms; but on receiving a form, it is terminated by that one. Again, form is made finite by matter, inasmuch as form, considered in itself, is common to many; but when received in matter, the form is determined to this one particular thing. Now matter is perfected by the form by which it is made finite; therefore infinite as attributed to matter, has the nature of something imperfect; for it is as it were formless matter. On the other hand, form is not made perfect by matter, but rather is contracted by matter; and hence the infinite, regarded on the part of the form not determined by matter, has the nature of something perfect. Now being is the most formal of all things, as appears from what is shown above (4, 1, Objection 3). Since therefore the divine being is not a being received in anything, but He is His own subsistent being as was shown above (Question 3, Article 4), it is clear that God Himself is infinite and perfect.
From this appears the Reply to the First Objection.
Reply to Objection 2. Quantity is terminated by its form, which can be seen in the fact that a figure which consists in quantity terminated, is a kind of quantitative form. Hence the infinite of quantity is the infinite of matter; such a kind of infinite cannot be attributed to God; as was said above, in this article.
Reply to Objection 3. The fact that the being of God is self-subsisting, not received in any other, and is thus called infinite, shows Him to be distinguished from all other beings, and all others to be apart from Him. Even so, were there such a thing as a self-subsisting whiteness, the very fact that it did not exist in anything else, would make it distinct from every other whiteness existing in a subject.
Source: Summa Theologica (New Advent)