Be you therefore perfect, as also your heavenly Father is perfect. Matthew 5:49 DR
Aquinas writes this below, on God as Perfect.
All created perfections are in God. Hence He is spoken of as universally perfect, because He lacks not (says the Commentator, Metaph. v) any excellence which may be found in any genus. This may be seen from two considerations. First, because whatever perfection exists in an effect must be found in the effective cause: either in the same formality, if it is a univocal agent---as when man reproduces man; or in a more eminent degree, if it is an equivocal agent---thus in the sun is the likeness of whatever is generated by the sun's power. Now it is plain that the effect pre-exists virtually in the efficient cause: and although to pre-exist in the potentiality of a material cause is to pre-exist in a more imperfect way, since matter as such is imperfect, and an agent as such is perfect; still to pre-exist virtually in the efficient cause is to pre-exist not in a more imperfect, but in a more perfect way. Since therefore God is the first effective cause of things, the perfections of all things must pre-exist in God in a more eminent way. Dionysius implies the same line of argument by saying of God (Div. Nom. v): "It is not that He is this and not that, but that He is all, as the cause of all." Secondly, from what has been already proved, God is existence itself, of itself subsistent (Question [3], Article [4]). Consequently, He must contain within Himself the whole perfection of being. For it is clear that if some hot thing has not the whole perfection of heat, this is because heat is not participated in its full perfection; but if this heat were self-subsisting, nothing of the virtue of heat would be wanting to it. Since therefore God is subsisting being itself, nothing of the perfection of being can be wanting to Him. Now all created perfections are included in the perfection of being; for things are perfect, precisely so far as they have being after some fashion. It follows therefore that the perfection of no one thing is wanting to God. This line of argument, too, is implied by Dionysius (Div. Nom. v), when he says that, "God exists not in any single mode, but embraces all being within Himself, absolutely, without limitation, uniformly;" and afterwards he adds that, "He is the very existence to subsisting things."
In this sense, perfection is a wholeness, a completeness of all that is good. God is not wanting in anything. And, all that is rests in Him. Nothing that is created exists outside of God. God, as the First Principle, is Perfect.
When God calls us to perfection, He is calling us to be all that He intended us to be. This cannot be done without His grace. This is the reason for purgation, to be emptied of all that stands in the way of our perfection in God.
God is the Essential Being and we participate in that. And, as we are made in the 'image and likeness of God", the image being free will and intellect, and the likeness being grace, which we must regain in baptism, we participate in the perfection of God according to how God has called us to that perfection.
God alone is totally Perfect.
http://www.newadvent.org/summa/1004.htm
