...how much he must suffer...

But the Lord said to him, “Go, for he is a chosen instrument of mine to carry my name before the Gentiles and kings and the children of Israel.

For I will show him how much he must suffer for the sake of my name.”

Acts 9:15-16

Jesus called Saul away from being a persecutor of Christians and into becoming the foremost preacher of the gospel in the New Testament. Jesus called Ananias to be the first Christian to come alongside Saul and to heal him from blindness. We know it was Jesus in verse 17, where Ananias reminds Saul that Jesus is Who met him on the Damascus road, and Jesus sent Ananias to restore Saul's sight. 

Jesus had plans for Saul. Jesus chose him, just like all the other apostles to send him before "Gentiles, kings, and the children of Israel". That commission from Jesus serves as a biographical summary of Saul's life from that point onward and it shows us an outline of the direction that the book of Acts will go from that point on in the storyline.

But there is an ironic twist in Saul's calling. He must also surely suffer greatly for Jesus. The man who once led the persecution of Christians with fierce and unrelently intensity would change to become the one most persecuted for Jesus' sake. He would endure unexpected perils and pains for the sake of the gospel. He would admonish others to do the same. Indeed, Saul's life from that point on would be characterized by both advancing the gospel with wild success into new regions and simultaneously suffering for it. While called to minister the good news of salvation, he was also called to endure hardship along with it.