Paul entered the synagogue and spoke boldly there for three months, arguing persuasively about the kingdom of God. But some of them became obstinate; they refused to believe and publicly maligned the Way. So Paul left them. He took the disciples with him and had discussions daily in the lecture hall of Tyrannus. This went on for two years, so that all the Jews and Greeks who lived in the province of Asia heard the word of the Lord.
Acts 19:8-10 New International Version (NIV)
Who is a better witness for what God can do than one whose life God has changed? Paul is an example of a nonbeliever of Christ transformed into a fervent, tireless disciple who worked diligently to spread the Gospel he had previously scoffed. Those who had followed Jesus knew of Saul/Paul's dogmatic persecution of believers in His way. It was difficult for them to trust him but Jesus was speaking to their hearts just as He had spoken to Paul. God had work to be done that only one transformed from total disbelief could do. If God would allow His only begotten Son to die on a cross for the sins of the world, it is clear He will go to great lengths to exact His plan of redemption and get His message across. With Paul, He did just that.
Often those who come to faith in Jesus Christ after living in disbelief are ridiculed for the change this new life will display. It doesn't matter. The angels rejoice as another joins the fold. Many did not believe Jesus was the Messiah sent from God and literally ridiculed Him to the death He overcame. However, God did not create us to question what He does. We who believe are to testify of the changes in our own lives since we accepted Christ as Savior and Lord, and hopefully these changes will be reflected in our behavior. The very act of doubting the authenticity of another's salvation is judgment and counts against those who do it. We are to be discerning, but Jesus Himself will reveal what comes next since He is our only righteous Judge. We are to pray for the presence and power of God to be made clear and truly take hold of them and us.
This business of God's is under His control and He will perform it. Like Paul, we can all be changed into vessels that God can use to further His Kingdom. Since the harvest is ripe, but the laborers are so few, God can choose anybody He created to do His work and He created everybody. He can change anyone who accepts His calling on their lives.
For the Lord is the Spirit, and wherever the Spirit of the Lord is, there is freedom. So all of us who have had that veil removed can see and reflect the glory of the Lord. And the Lord—who is the Spirit—makes us more and more like him as we are changed into his glorious image. 2 Corinthians 3:17-18 NLT