Acts 9:15 But the Lord said unto him, Go thy way: for he is a chosen vessel unto Me, to bear My name before the Gentiles, and kings, and the children of Israel:
Does this say that Paul was a volunteer for the service of the church? No, God said, "He is a chosen vessel unto me," meaning that Paul was one of God's elect. Paul had gained the experience to serve a purpose for God, and God knew Paul from the beginning. From before even the foundations of the world [this earth age of the flesh].
Paul was selected by God to bear the name of Jesus Christ the Son of God, before the Gentiles, the kings, which are those in authority, and the children of Israel. Paul had a three-fold mission. Paul, therefore, taught on three levels, and it is for this reason that any true teacher, gifted in God's Word, will also teach on three levels. These levels mean that wherever one person is in the Word of God, there will be a message in it for them.
As those people grow and mature in the Word, they could listen to that same teacher three times and each time through, they should learn something new each time. They will think that it is a new discovery each time through, for God's Word builds on itself, and what is important to the baby in Christ, seeking the milk of the Word, is different than the mature Christian that seeks more than just salvation and baptism. A gifted teacher, such as Paul, teaches on whatever level the listener is at. That is why it is important that you learn from the Word of God, and not the words and traditions of men.
Notice there is a difference between those men of free will, and those men that have a destiny, and purpose of the elect of God.
Those of the elect know deep inside themselves that they have a destiny and purpose for their lives.
Paul knew he had a destiny, only he was confused which way that was. He grabbed out at man's way, instead of seeking God's way. When the time came, God put a stop to Paul's way, and He blinded him.