1 John 1:6

If we say that we have fellowship uith him - If we reckon ourselves among his friends, or, in other words, if we profess to be like him: for a profession of religion involves the idea of having fellowship with God, (compare the notes at 1 John 1:3), and he who professes that should be like him.

And walk in darkness - Live in sin and error. To “walk in darkness” now commonly denotes to be in doubt about our religious state, in contradistinction from living in the enjoyment of religion. That is not, however, probably the whole idea here. The leading thought is, that if we live in sin, it is a proof that our profession of religion is false. Desirable as it is to have the comforts of religion, yet it is not always true that they who do not are not true Christians, nor is it true by any means that they intend to deceive the world.

We lie - We are false professors; we are deceived if we think that we can have fellowship with God, and yet live in the practice of sin. As God is pure, so must we be, if we would be his friends. This does not mean necessarily that they meant to deceive, but that there was an irreconcilable contradiction between a life of sin and fellowship with God.

And do not the truth - Do not act truly. The profession is a false one. Compare the notes at John 3:22. To do the truth is to act in accordance with truth; and the expression here means that such an one could not be a Christian. And yet how many there are who are living in known sin who profess to be Christians! How many whose minds are dark on the whole subject of religion, who have never known anything of the real peace and joy which it imparts, who nevertheless entertain the belief that they are the friends of God, and are going to heaven! They trust in a name, in forms, in conformity to external rites, and have never known anything of the internal peace and purity which religion imparts, and in fact have never had any true fellowship with that God who is light, and in whom there is no darkness at all. Religion is light; religion is peace, purity, joy; and though there are eases where for a time a true Christian may be left to darkness, and have no spiritual joy, and be in doubt about his salvation, yet still it is a great truth, that unless we know by personal experience what it is to walk habitually in the light, to have the comforts of religion, and to experience in our own souls the influences which make the heart pure, and which bring us into conformity to the God who is light, we can have no true religion. All else is but a name, which will not avail us on the final day.