The interpretation timeline
1 John 2:9 Douay-Rheims

He that saith he is in the light, and hateth his brother, is in darkness even until now.

A.D. 215 – 1871 7 witnesses 8 excerpts

Patristic

before A.D. 750
215
A.D.
c. A.D. 150–215
“"He that saith he is in the light,"-in the light, he means in the truth,-"and hateth," he says, "his brother." By his brother, he means not only his neighbour, but also the Lord. For unbelievers hate Him and do not keep His commandments. Therefore also he infers: "He that loveth his brother abideth in the light; and there is none occasion of stumbling in him."”
Source
258
A.D.
Cyprian Patristic
c. A.D. 200–258
“That charity and brotherly affection are to be religiously and stedfastly practised. In Malachi: "Hath not one God created us? Is there not one Father of us all? Why have ye certainly deserted every one his brother? " Of this same thing according to John: "Peace I leave with you, my peace I give unto you." Also in the same place: "This is my commandment, That ye love one another, even as I have loved you. Greater love than this has no man, than that one should lay down his life for his friends." Also in the same place: "Blessed are the peacemakers, for they shall be called the sons of God." Also in the same place: "Verily I say unto you, That if two of you shall agree on earth concerning everything, whatever you shall ask it shall be given you from my Father which is in heaven. For wherever two or three are gathered together in my name, I am with them." Of this same thing in the first Epistle to the Corinthians: "And I indeed, brethren, could not speak unto you as to spiritual, but as to carnal, as to babes in Christ. I have given you milk for drink, not meat: for while ye were yet little ye were not able to bear it, neither now are ye able. For ye are still carnal: for where there are in you emulation, and strife, and dissensions, are ye not carnal, and walk after man? " Likewise in the same place: "And if I should have all faith, so that I can remove mountains, but have not charity, I am nothing. And if I should distribute all my goods for food, and if I should deliver up my body to be burned, but have not charity, I avail nothing. Charity is great-souled; charity is kind; charity envieth not; charity dealeth not falsely; is not puffed up; is not irritated; thinketh not evil; rejoiceth not in injustice, but rejoiceth in the truth. It loveth all things, believeth all things, hopeth all things, beareth all things. Charity shall never fail." Of this same thing to the Galatians: "Thou shalt love thy neighbour as thyself. But if ye bite and accuse one another, see that ye be not consumed one of another." Of this same thing in the Epistle of John: "In this appear the children of God and the children of the devil. Whosoever is not righteous is not of God, and he who loveth not his brother. For he who hateth his brother is a murderer; and ye know that no murderer hath eternal life abiding in him." Also in the same place: "If any one shall say that he loves God, and hates his brother, he is a liar: for he who loveth not his brother whom he seeth, how can he love God whom he seeth not? " Of this same thing in the Acts of the Apostles: "But the multitude of them that had believed acted with one soul and mind: nor was there among them any distinction, neither did they esteem as their own anything of the possessions that they had; but all things were common to them." Of this same thing in the Gospel according to Matthew: If thou wouldest offer thy gift at the altar, and there rememberest that thy brother hath ought against thee; leave thou thy gift before the altar, and go; first be reconciled to thy brother, and then come and offer thy gift at the altar." Also in the Epistle of John: "God is love l and he that dwelleth in love dwelleth in God, and God in him." Also in the same place: "He who saith he is in the light, and hateth his brother, is a liar, and walketh in darkness even until now."”
Source
“Why do you rush into the darkness of jealousy? why do you enfold yourself in the cloud of malice? why do you quench all the light of peace and charity in the blindness of envy? why do you return to the devil, whom you had renounced? why do you stand like Cain? For that he who is jealous of his brother, and has him in hatred, is bound by the guilt of homicide, the Apostle John declares in his epistle, saying, "Whosoever hateth his brother is a murderer; and ye know that no murderer hath life abiding in him."24 And again: "He that saith he is in the light, and hateth his brother, is in darkness even until now, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes."25 Whosoever hates, says he, his brother, walketh in darkness, and knoweth not whither he goeth. For he goeth unconsciously to Gehenna, in ignorance and blindness; he is hurrying into punishment, departing, that is, from the light of Christ, who warns and says, "I am the light of the world. He that followeth me shall not walk in darkness, but shall have the light of life."26 But he follows Christ who stands in His precepts, who walks in the way of His teaching, who follows His footsteps and His ways, who imitates that which Christ both did and taught; in accordance with what Peter also exhorts and warns, saying, "Christ suffered for us, leaving you an example that ye should follow His steps."27”
Source
430
A.D.
Augustine of Hippo Patristic
A.D. 354–430
“"He that saith he is in the light, and hateth his brother, is in darkness even until now." Now he is making all clear that he has been saying. What! my brethren, how long shall we say to you, "Love your enemies"? See whether, what is worse, ye do not hate your brethren. If ye loved only your brethren, ye would be not yet perfect: but if ye hate your brethren, what are ye, where are ye? Let each look to his own heart: let him not keep hatred against his brother for any hard word; on account of earthly contention let him not become earth. For whoso hates his brother, let him not say that he walks in the light. "He that saith he is in the light, and hateth his brother, is in darkness even until now." Thus, some man who was a pagan has become a Christian; mark well: behold he was in darkness, while he was a pagan: now is he made henceforth a Christian; thanks be to God, say all joyfully; the apostle is read, where he saith joyfully, "For ye were sometime darkness, but now light in the Lord." Once he worshipped idols, now he worships God; once he worshipped the things he made, now he worships Him that made him. He is changed: thanks be to God, say all Christians with joyful greeting. Why? Because henceforth he is one that adores the Father and the Son and the Holy Ghost; one that detests demons and idols. Yet still is John solicitous about our convert: while many greet him with joy, by him he is still looked upon with apprehension. Brethren, let us gladly welcome a mother's solicitude. Not without cause is the mother solicitous about us when others rejoice: by the mother, I mean charity: for she dwelt in the heart of John, when he spake these words. Wherefore, but because there is something he fears in us, even when men now hail us with joy? What is it that he fears? "He that saith he is in the light" - What is this? He that saith now he is a Christian, - "and hateth his brother, is in darkness even until now."”
Source

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“He that saith he is in the light,.... Is in Christ the light, or has the true knowledge of the light of the Gospel, or is illuminated by the Spirit of God; for persons may profess to be enlightened ones, and not be so: wherefore the apostle does not say, he that is in the light, but he that says he is, and hateth his brother; who is so either by creation, as all men are brethren, having one Father, that has made them, and brought them up; or by regeneration, being born of God the Father, and in the same family and household of faith; and so regards such who are in a spiritual relation, whom to hate internally, or not to love, is inconsistent with being in the light, or having faith, which is always naturally and necessarily accompanied with the heat of love; for as light and heat, so faith and love go together: wherefore, let a man's profession of light be what it will, if love to his brother is wanting, he is in darkness even until now; he is in a state of nature and unregeneracy, which is a state of darkness and ignorance; he is under the power of darkness, and in the kingdom of Satan; who is the ruler of the darkness of this world; he ever was so from his birth; he never was called nor delivered out of it, but is still in it to this moment, and so remains. This seems to be very much levelled against the Jews, who make hatred of the brother in some cases lawful: for they say (d), "if one man observes sin in another, and reproves him for it, and he does not receive his reproof, , "it is lawful to hate him";'' See Gill on Mat 5:43. (d) Moses Kotsensis Mitzvot Tora, pr. neg. 5.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“He that saith he is in the light - He that professes to be a convert to Christianity, even in the lowest degree; and hateth his brother - not only does not love him, but wills and does him evil, as the Jews did the Gentiles; is in darkness - has received no saving knowledge of the truth; and, whatever he may pretend, is in heathen ignorance, or even worse than heathen ignorance, to the present time, notwithstanding the clear shining of the light of the Gospel.”
Source
1871
A.D.
1871
“There is no mean between light and darkness, love and hatred, life and death, God and the world: wherever spiritual life is, however weak, there darkness and death no longer reign, and love supplants hatred; and Luk 9:50 holds good: wherever life is not, there death, darkness, the flesh, the world, and hatred, however glossed over and hidden from man's observation, prevail; and Luk 11:23 holds good. "Where love is not, there hatred is; for the heart cannot remain a void" [BENGEL]. in the light--as his proper element. his brother--his neighbor, and especially those of the Christian brotherhood. The very title "brother" is a reason why love should be exercised. even until now--notwithstanding that "the true light already has begun to shine" (Jo1 2:8).”
Source

Undated

date unknown
date
unknown
Glossa Ordinaria Medieval
“He who says, etc. He determines what commandment he takes up, that is, charity, and in what manner that charity itself is to be had, namely the love of God and of neighbour.”
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.