If we say that we have fellowship with him and walk in the darkness, we lie, and don't tell the truth.
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.