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!
10. “He that does sin, is of the devil, because the devil sins from the beginning.” “Is of the devil:” ye know what he means: by imitating the devil. For the devil made no man, begot no man, created no man: but whoso imitates the devil, that person, as if begotten of him, becomes a child of the devil; by imitating him, not literally by being begotten of him. In what sense are you a child of Abraham, not that Abraham begot you? In the same sense as the Jews, the children of Abraham, not imitating the faith of Abraham, have become children of the devil: of the flesh of Abraham they were begotten, and the faith of Abraham they have not imitated. If then those who were thence begotten were put out of the inheritance, because they did not imitate, you, who art not begotten of him, art made a child, and in this way shall be a child of him by imitating him. And if you imitate the devil, in such wise as he became proud and impious against God, you will be a child of the devil: by imitating, not that he created you or begot you.
11. “Unto this end was the Son of God manifested.” Now then, brethren, mark! All sinners are begotten of the devil, as sinners. Adam was made by God: but when he consented to the devil, he was begotten of the devil; and he begot all men such as he was himself. With lust itself we were born; even before we add our sins, from that condemnation we have our birth. For if we are born without any sin, wherefore this running with infants to baptism that they may be released? Then mark well, brethren, the two birth-stocks, Adam and Christ: two men are; but one of them, a man that is man; the other, a Man that is God. By the man that is man we are sinners; by the Man that is God we are justified. That birth has cast down unto death; this birth has raised up unto life: that birth brings with it sin; this birth sets free from sin. For to this end came Christ as Man, to undo the sins of men. “Unto this end was the Son of God manifested, that He may undo the works of the devil.”
Source: Homilies on the First Epistle of John (New Advent)