2 Timothy 1:3 KJV
I thank God, whom I serve from my forefathers with pure conscience, that without ceasing I have remembrance of thee in my prayers night and day;
The Apostle had once been an aggressive persecutor of Christians, committing many to prison and commending some to death. And yet here he says he has served God with a pure conscience "from my forefathers."
I see at least two possible applications to this passage:
His service since salvation has been based upon the authority of the forefathers of faith
Abraham Isaac and Jacob had no part in the malicious thing Judaism had become in Paul's day. When he got saved, his new service sprang from that faith which traced to Abraham and not from that faith which had become twisted out of Abraham.
His salvation had cleansed his conscience of those things previous to it
Salvation liberated him from the burden of dead works to serve the Lord with a new and pure conscience. His salvation erased the stain of a corrupt religion and restored in him the original faith of Abraham wherein God imputed unto him righteousness.
And so we may serve God with pure consciences. We need never live under the burden of the past but ever look forward. Through Christ, whatever our past holds is buried and it is as if we have been pure since the days of the very first believers.