Objection 1. It seems that perfection does not belong to God. For we say a thing is perfect if it is completely made. But it does not befit God to be made. Therefore He is not perfect.
Objection 2. Further, God is the first beginning of things. But the beginnings of things seem to be imperfect, as seed is the beginning of animal and vegetable life. Therefore God is imperfect.
Objection 3. Further, as shown above (Question 3, Article 4), God's essence is existence. But existence seems most imperfect, since it is most universal and receptive of all modification. Therefore God is imperfect.
On the contrary, It is written: "Be you perfect as also your heavenly Father is perfect" (Matthew 5:48).
I answer that, As the Philosopher relates (Metaph. xii), some ancient philosophers, namely, the Pythagoreans and Leucippus, did not predicate "best" and "most perfect" of the first principle. The reason was that the ancient philosophers considered only a material principle; and a material principle is most imperfect. For since matter as such is merely potential, the first material principle must be simply potential, and thus most imperfect. Now God is the first principle, not material, but in the order of efficient cause, which must be most perfect. For just as matter, as such, is merely potential, an agent, as such, is in the state of actuality. Hence, the first active principle must needs be most actual, and therefore most perfect; for a thing is perfect in proportion to its state of actuality, because we call that perfect which lacks nothing of the mode of its perfection.
Reply to Objection 1. As Gregory says (Moral. v, 26,29): "Though our lips can only stammer, we yet chant the high things of God." For that which is not made is improperly called perfect. Nevertheless because created things are then called perfect, when from potentiality they are brought into actuality, this word "perfect" signifies whatever is not wanting in actuality, whether this be by way of perfection or not.
Reply to Objection 2. The material principle which with us is found to be imperfect, cannot be absolutely primal; but must be preceded by something perfect. For seed, though it be the principle of animal life reproduced through seed, has previous to it, the animal or plant from which is came. Because, previous to that which is potential, must be that which is actual; since a potential being can only be reduced into act by some being already actual.
Reply to Objection 3. Existence is the most perfect of all things, for it is compared to all things as that by which they are made actual; for nothing has actuality except so far as it exists. Hence existence is that which actuates all things, even their forms. Therefore it is not compared to other things as the receiver is to the received; but rather as the received to the receiver. When therefore I speak of the existence of man, or horse, or anything else, existence is considered a formal principle, and as something received; and not as that which exists.
Source: Summa Theologica (New Advent)