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How long did Abraham not speak or have intimate fellowship with God? What about Solomon before he wrote Proverbs? Did the disciples break fellowship with Christ when they forsook him at the cross and fled? Did the 70 disciples who went back and followed him no more break their fellowship with Christ? I suppose none of them believed He was the Christ. What about Titus who left Paul and some other disciples that distanced themselves from the doctrine that Paul taught like John Mark? Are you going to tell us that if any of them returned then they will be saved but if not then they would lose their salvation? David asked God to restore unto him the JOY of his salvation and not THE salvation he had already. David never doubted his salvation no matter how much sin was before him of his own life.
That is because David continued in obedience by confessing his sins, that is why he never had doubt.
The scriptures make it clear that not all will continue in fellowship with the Lord, and will turn away and stay separated from Him. That they will never come back in repentance because they love darkness more then light.
John 17:12
While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
John 6:66
From that time many of his disciples went back, and walked no more with him.
Matthew 24:10-13
And then many will fall away and betray one another and hate one another. And many false prophets will arise and lead many astray. And because lawlessness will be increased, the love of many will grow cold. But the one who endures to the end will be saved.
Hebrews 3:12-14
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. For we have come to share in Christ, if indeed we hold our original confidence firm to the end.