God's plan for appointed witnesses to testify of Him and His salvation constitutes a cycle. God starts things by the many witnesses He has sent out to confront us all concerning our sin problem before it is too late. Out of those who hear God's message of salvation, some come to Christ. Those who believe are then appointed to take their turn as God's witnesses. Who would be a more reliable witness of God's saving power than those who have personally received it?
The maniac of Gadara in Mark 5 lived a life of madness and self-ruin by the power over him of many devils. Jesus set him free and forgave all his sin. It was the love of God that then commanded him to go back and tell all his friends what the Lord could do for them too.
This plan of God is also the explanation in part for baptism. God has established a simple object lesson whereby one testifies that he has received the saving work of Jesus Christ personally. In baptism, being dipped in water witnesses to the death, burial, and resurrection of Jesus Christ to provide salvation from sin. It is a powerful way to tell others you have trusted the sacrifice of Jesus Christ as payment for your sins.
Further, God requires those who are forgiven to demonstrate their salvation from sin by putting sin out of their lives. "For the grace of God that bringeth salvation hath appeared to all men, Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world" (Titus 2:11-12).
Even the Bible believing local church is part of God's plan for Christian testimony. Nothing could be more obvious from the New Testament. The church's responsibility is to hold up the truth in the community, and those who are saved by faith are appointed by God to take their place in it. Paul describes it as "the pillar and ground of the truth" in I Timothy 3:15.