David committed adultery with Uriah's wife, Bathsheba and she became pregnant. He called Uriah from the battlefield (where David should have been), got him drunk so he would sleep with her, but Uriah refused to sleep with his wife while he had men on the battlefield, so David sent a message to have the men pull back in battle and leave Uriah to be killed by the enemy. Then he married Bathsheba. He was so convicted and distraught with himself he called upon the Lord to forgive him and to restore, not his salvation, but the joy of his salvation in Psalm 51. He did not lose his salvation, although he did suffer the consequences of his actions for years to come. I have an acquaintance who believes in the Lord. Many years ago, in his youth he sinned by drinking (he was drunk) and driving. He wrecked and lost half a leg and has a big metal piece up his forearm and other damage to his head and ears and whatnot. He admitted he sinned and believes in the Lord and he is saved, however, he will go through life without that leg.
If one can see that salvation is an event, where we see we are sinners and trust in Christ to have paid for those sins and offers salvation freely, at that moment we who were dead in sin are made alive in Him. There may have been a process that lead up to that event, and there is a process of sanctification after that event, but receiving the free gift of salvation by grace through faith in Christ is a one time event.
I am Alpha and Omega, the beginning and the end. I will give unto him that is athirst of the fountain of the water of life freely. Rev. 21:6
And the Spirit and the bride say, Come. And let him that heareth say, Come. And let him that is athirst come. And whosoever will, let him take the water of life freely. Rev. 22:17
Thus saith the LORD, In an acceptable time have I heard thee, and in a day of salvation have I helped thee: and I will preserve thee, Isaiah 49:8a (notice he saves and keeps/preserves)
(For he saith, I have heard thee in a time accepted, and in the day of salvation have I succoured thee: behold, now is the accepted time; behold, now is the day of salvation.) 2 Cor. 6:2
and the same day there were added unto them about three thousand souls. Acts 2:41b