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

People's New Testament

1:6 If we... walk in darkness. To claim a fellowship with him who is Light and to walk in the darkness of error, of ignorance, and of sinfulness are utterly inconsistent.

Do not the truth. He who obeys not the truth walks in darkness.

Wesley's Notes

1:6 If we say - Either with our tongue, or in our heart, if we endeavour to persuade either ourselves or others. We have fellowship with him, while we walk, either inwardly or outwardly, in darkness - In sin of any kind. We do not the truth - Our actions prove, that the truth is not in us.

Jamieson-Fausset-Brown Bible Commentary

6. say-profess.

have fellowship with him-(1Jo 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 1Jo 2:8, 11).

lie-(1Jo 2:4).

do not-in practice, whatever we say.

the truth-(Eph 4:21; Joh 3:21).

Matthew Henry's Concise Commentary

1:5-10 A message from the Lord Jesus, the Word of life, the eternal Word, we should all gladly receive. The great God should be represented to this dark world, as pure and perfect light. As this is the nature of God, his doctrines and precepts must be such. And as his perfect happiness cannot be separated from his perfect holiness, so our happiness will be in proportion to our being made holy. To walk in darkness, is to live and act against religion. God holds no heavenly fellowship or intercourse with unholy souls. There is no truth in their profession; their practice shows its folly and falsehood. The eternal Life, the eternal Son, put on flesh and blood, and died to wash us from our sins in his own blood, and procures for us the sacred influences by which sin is to be subdued more and more, till it is quite done away. While the necessity of a holy walk is insisted upon, as the effect and evidence of the knowledge of God in Christ Jesus, the opposite error of self-righteous pride is guarded against with equal care. All who walk near to God, in holiness and righteousness, are sensible that their best days and duties are mixed with sin. God has given testimony to the sinfulness of the world, by providing a sufficient, effectual Sacrifice for sin, needed in all ages; and the sinfulness of believers themselves is shown, by requiring them continually to confess their sins, and to apply by faith to the blood of that Sacrifice. Let us plead guilty before God, be humble, and willing to know the worst of our case. Let us honestly confess all our sins in their full extent, relying wholly on his mercy and truth through the righteousness of Christ, for a free and full forgiveness, and our deliverance from the power and practice of sin.

Matthew Henry's Whole Bible Commentary

Verses 5-7

The apostle, having declared the truth and dignity of the author of the gospel, brings a message or report from him, from which a just conclusion is to be drawn for the consideration and conviction of the professors of religion, or professed entertainers of this glorious gospel.

I. Here is the message or report that the apostle avers to come from the Lord Jesus: This then is the message which we have heard of him (v. 5), of his Son Jesus Christ. As he was the immediate sender of the apostles, so he is the principal person spoken of in the preceding context, and the next antecedent also to whom the pronoun him can relate. The apostles and apostolical ministers are the messengers of the Lord Jesus; it is their honour, the chief they pretend to, to bring his mind and messages to the world and to the churches. This is the wisdom and present dispensation of the Lord Jesus, to send his messages to us by persons like ourselves. He that put on human nature will honour earthen vessels. It was the ambition of the apostles to be found faithful, and faithfully to deliver the errands and messages they had received. What was communicated to them they were solicitous to impart: This then is the message which we have heard of him, and declare unto you. A message from the Word of life, from the eternal Word, we should gladly receive: and the present one is this (relating to the nature of God whom we are to serve, and with whom we should covet all indulged communion)-That God is light, and in him is no darkness at all, v. 5. This report asserts the excellency of the divine nature. He is all that beauty and perfection that can be represented to us by light. He is a self-active uncompounded spirituality, purity, wisdom, holiness, and glory. And then the absoluteness and fulness of that excellency and perfection. There is no defect or imperfection, no mixture of any thing alien or contrary to absolute excellency, no mutability nor capacity of any decay in him: In him is no darkness at all, v. 5. Or this report may more immediately relate to what is usually called the moral perfection of the divine nature, what we are to imitate, or what is more directly to influence us in our gospel work. And so it will comprehend the holiness of God, the absolute purity of his nature and will, his penetrative knowledge (particularly of hearts), his jealousy and injustice, which burn a a most bright and vehement flame. It is meet that to this dark world the great God should be represented as pure and perfect light. It is the Lord Jesus that best of all opens to us the name and nature of the unsearchable God: The only-begotten, who is in the bosom of the Father, the same hath declared him. It is the prerogative of the Christian revelation to bring us the most noble, the most august and agreeable account of the blessed God, such as is most suitable to the light of reason and what is demonstrable thereby, most suitable to the magnificence of his works round about us, and to the nature and office of him that is the supreme administrator, governor, and judge of the world. What more (relating to and comprehensive of all such perfection) could be included in one word than in this, God is light, and in him is no darkness at all? Then,

II. There is a just conclusion to be drawn from this message and report, and that for the consideration and conviction of professors of religion, or professed entertainers of this gospel. This conclusion issues into two branches:-1. For the conviction of such professors as have no true fellowship with God: If we say we have fellowship with him, and walk in darkness, we lie, and do not the truth. It is known that to walk, in scripture account, is to order and frame the course and actions of the moral life, that is, of the life so far as it is capable of subjection to the divine law. To walk in darkness is to live and act according to such ignorance, error, and erroneous practice, as are contrary to the fundamental dictates of our holy religion. Now there may be those who may pretend to great attainments and enjoyments in religion; they may profess to have communion with God; and yet their lives may be irreligious, immoral, and impure. To such the apostle would not fear to give the lie: They lie, and do not the truth. They belie God; for he holds no heavenly fellowship or intercourse with unholy souls. What communion hath light with darkness? They belie themselves, or lie concerning themselves; for they have no such communications from God nor accesses to him. There is no truth in their profession nor in their practice, or their practice gives their profession and pretences the lie, and demonstrates the folly and falsehood of them. 2. For the conviction and consequent satisfaction of those that are near to God: But, if we walk in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. As the blessed God is the eternal boundless light, and the Mediator is, from him, the light of the world, so the Christian institution is the great luminary that appears in our sphere, and shines here below. A conformity to this in spirit and practice demonstrates fellowship or communion with God. Those that so walk show that they know God, that they have received of the Spirit of God, and that the divine impress or image is stamped upon their souls. Then we have fellowship one with another, they with us and we with them, and both with God, in his blessed or beatific communications to us. And this is one of those beatific communications to us-that his Son's blood or death is applied or imputed to us: The blood of Jesus Christ his Son cleanseth us from all sin. The eternal life, the eternal Son, hath put on flesh and blood, and so became Jesus Christ. Jesus Christ hath shed his blood for us, or died to wash us from our sins in his own blood. His blood applied to us discharges us from the guilt of all sin, both original and actual, inherent and committed: and so far we stand righteous in his sight; and not only so, but his blood procures for us those sacred influences by which sin is to be subdued more and more, till it is quite abolished, Gal. 3:13, 14.