Who can say, “I have cleansed my heart, I am pure from my sin”? Proverbs 20:9
True manifestation of those who have believed at heart is repentance from their sins. It bears testimony that Jesus is real in their lives, and not something momentary, which would pass and no longer be remembered. They strive to be better, their lives being reflections of Christ, and they let nothing stain their clothing. They know what sin is, but also to be free from it.
In addition, they want to be an example for those with whom they walked, together in trespasses and sins, so they can see the change Jesus made in their lives. They show them that one can change indeed, that it only takes believing and making a decision.
These signs are the ones that show that there is conviction of sin and one is grateful to God for what He has done. He brought us out from all this uncleanliness and wiped all past, through his precious blood that was shed on the cross of Calvary. It is for all those who want to get out of that condition of life and live in holiness.