1 John 1:6. 'If we say that we have fellowship with Him, and walk in darkness, we lie and do not practise the truth.'
First of all, John is uniting himself with his readers: 'If we say.' Secondly, it is a conditional sentence, and the Greek verb eipomen, 'say,' is in the subjunctive. 'If we should say.....' or 'If we were to say.....' Thirdly, eipomen is in the aorist tense, meaning a single action, but the second verb, peripatomen is present subjunctive, denoting continuous action. Koinonian echomen is in the present indicative, again indicating continuous action. So if John and/or his readers were to say that they have continuous fellowship with God while they are consistently walking in darkness, then they are lying and not practising the truth.
The suggestion here, because of the subjunctive mood, is that John and his readers are not walking in darkness, and that therefore they are saved. But if they were to commence walking in darkness, not just briefly, but consistently, they would be calling their salvation into question unless the Lord led them to repentance. It is possible for one of the Lord's sheep to become a lost sheep, and to find itself walking in spiritual darkness for a period. But if it does so, the Good Shepherd will leave His ninety and nine sheep and search out and save the one that was lost. However, if He doesn't do that, it rather suggests that the sheep in question does not belong to the Lord and will eventually hear Him say, "I never knew you! Depart from Me you who practise lawlessness."
Note this: "I never knew you!" Not, "I knew you once and then disowned you." David walked in darkness for a period in his life, but the Lord brought him back through the prophet Nathan, so he could truly say, "The LORD is my Shepherd."