All of us have a past. All of us have sinned and continue to be sinners. It is only because of God's grace and His wonderful mercy that I have hope of eternity with Him.
Each of us has someone to thank for bringing us to Christ. Many of us need to be thankful for those that help keep us on the right path. And all of us need to be that person for others. When you see someone outside of Christ, don't shake your head and tsk tsk and walk away. Maybe they are waiting to be found. Maybe they need to meet Jesus through you.