17 The context of this statement in 1:6 indicates clearly that the progressive (continuative or durative) aspect of the present tense must be in view here. The relationship of the phrase keep on walking to if we say is very important for understanding the problem expressed in 1:6. If one should say (εἴπωμεν, eipōmen) that he has fellowship with God, and yet continues walking (περιπατῶμεν, peripatōmen) in the darkness, then it follows (in the apodosis, the “then” clause) that he is lying and not practicing the truth.
18 Or “living according to …”

In other versions

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

If we claim to have fellowship with him and yet walk in the darkness, we lie and do not live out the truth.

If we say, “We have fellowship with him,” and yet we walk in darkness, we are lying and are not practicing the truth.

So we are lying if we say we have fellowship with God but go on living in spiritual darkness; we are not practicing the truth.

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

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

Public Domain