9 “Little children, let no man seduce you. He that does righteousness is righteous, as He is righteous.” What, on hearing that we are “righteous as He is righteous,” are we to think ourselves equal with God? You must know what means that “as:” thus he said a while ago, “Purifies himself even as He is pure.” Then is our purity like and equal to the purity of God, and our righteousness to God's righteousness? Who can say this? But the word “as,” is not always wont to be used in the sense of equality.
As, for example, if, having seen this large church, a person should wish to build a smaller church, but with the same relative dimensions: as, for example, if this be one measure in width and two measures in length, he too should build his church one measure in width and two measures in length: in that case one sees that he has built it “as” this is built. But this church has, say, a hundred cubits in length, the other thirty: it is at once “as” this, and yet unequal. You see that this “as” is not always referred to parity and equality.
For example, see what a difference there is between the face of a man and its image from a mirror: there is a face in the image, a face in the body: the image exists in imitation, the body in reality. And what do we say? Why, “as” there are eyes here, so also there; “as” ears here, so ears also there. The thing is different, but the “as” is said of the resemblance. Well then, we also have in us the image of God; but not that which the Son equal with the Father has: yet except we also, according to our measure, were “as” He, we should in no respect be said to be like Him.
“He purifies us,” then, “even as He is pure:” but He is pure from eternity, we pure by faith. We are “righteous even as He is righteous;” but He is so in His immutable perpetuity, we righteous by believing on One we do not see, that so we may one day see Him. Even when our righteousness shall be perfect, when we shall be equal to the angels, not even then shall it be equalled with Him. How far then is it from Him now, when not even then it shall be equal!
Source: Homilies on the First Epistle of John (New Advent)