Adam Clarke

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.