If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth.

“...the apostles who did also write the Gospel, in which they recorded the doctrine regarding God, pointing out that our Lord Jesus Christ is the truth, and that no lie is in Him. As also David says, prophesying His birth from a virgin, and the resurrection from the dead, "Truth has sprung out of the earth." The apostles, likewise, being disciples of the truth, are above all falsehood; for a lie has no fellowship with the truth, just as darkness has none with light, but the presence of the one shuts out that of the other. Our Lord, therefore, being the truth, did not speak lies...”

“And how can they say that they alone know God when they do the same things as those who are loathed by the heathen because they do not do what the laws direct, that is, as the wicked and incontinent and covetous and adulterous? They ought to live good lives even while they are dwelling in an alien country, to manifest their truly kingly nature. But because they have chosen to disobey the laws, they make themselves objects of hatred both to human lawgivers and to the law of God. At any rate in Numbers the man who thrust his spear into the fornicator is evidently blessed by God. And John says in his epistle: "If we say that we have fellowship with him and walk in darkness, we lie and do not the truth; but if we walk in the light as he is in the light, we have fellowship with him, and the blood of Jesus his Son cleanses us from sin." How then are they who do these things superior to worldly men when they behave like the very worst men of this world? Those whose actions are alike are in my opinion of like nature.”

“If "God be light, and in Him is no darkness at all, and we must have fellowship with Him," then from us also must the darkness be driven away, that there may be light created in us, for darkness cannot have fellowship with light. "If we say that we have fellowship with Him, and walk in darkness, we lie." Thou sayest thou hast fellowship with God, and thou walkest in darkness; "and God is light, and in Him is no darkness at all:" then how should there be fellowship between light and darkness? At this point therefore a man may say to himself, What shall I do? how shall I be light? I live in sins and iniquities. There steals upon him, as it were, a desperation and sadness. There is no salvation save in the fellowship of God. "God is light, and in Him is no darkness at all." But sins are darkness, as the Apostle saith of the devil and his angels, that they are "rulers of this darkness." He would not call them of darkness, save as rulers of sins, having lordship over the wicked.”

“If we say that we have fellowship with him,.... The Alexandrian copy reads, "for if we say": that is, if any profess to be partakers of the divine nature, to be like unto God, and to have communion with him, to have the light of his countenance, and the discoveries of his love: and walk in darkness; in the darkness of sin, ignorance, and unbelief, or are in a state of unregeneracy and blindness; whose understandings are darkened, and they know not God in Christ, nor have any true sight and sense of themselves, their sin and danger; and are ignorant of Christ and his righteousness, and the way of salvation by him; and are strangers to the Spirit of God, and the work of his grace; and are unacquainted with the truths of the Gospel; and not only so, but go on in darkness more and more; prefer it to the light, love it, and the works of it; have fellowship with them, and choose them; take pleasure in the ways of sin and wickedness, and continue, and walk on in them; if such persons pretend to fellowship with God, they are liars: we lie; it cannot be, it is a contradiction, the thing is impossible and impracticable; what communion hath light with darkness? or what fellowship can the throne of iniquity, or those in whom sin reigns, have with God? for God is light, and were they partakers of him, or like unto him, or had communion with him, they would consequently be in the light, and not in darkness, and much less walk in it; wherefore they are liars, and do not the truth: they do not say the truth, nor act according to it; they do not act uprightly or sincerely, but are hypocrites, and pretend to that which they have not; and if they did the truth, they would come to the light, and not walk in darkness; see Joh 3:21.”

“If we say that we have fellowship - Having fellowship, κοινωνια, communion, with God, necessarily implies a partaking of the Divine nature. Now if a man profess to have such communion, and walk in darkness - live an irreligious and sinful life, he lies, in the profession which he makes, and does not the truth - does not walk according to the directions of the Gospel, on the grace of which he holds his relation to God, and his communion with him. The Gnostics, against whose errors it is supposed this epistle was written, were great pretenders to knowledge, to the highest degrees of the Divine illumination, and the nearest communion with the fountain of holiness, while their manners were excessively corrupt.”

“say--profess. have fellowship with him-- (Jo1 1:3). The essence of the Christian life. walk--in inward and outward action, whithersoever we turn ourselves [BENGEL]. in darkness--Greek, "in the darkness"; opposed to "the light" (compare Jo1 2:8, Jo1 2:11). lie-- (Jo1 2:4). do not--in practice, whatever we say. the truth-- (Eph 4:21; Joh 3:21).”