Romans 1:1b

Paul, a servant of Christ Jesus, called to be an apostle and set apart for the gospel of God—

(v1, NIV)

Not only is Paul a servant of Christ Jesus, but he is also called to be an apostle. To be an apostle, you have to have seen the risen Christ and be commissioned by him.

Then he appeared to James, then to all the apostles, and last of all he appeared to me also, as to one abnormally born.

(1 Corinthians 15:7-8, NIV)

In chapter 15 of 1 Corinthians, Paul begins with a recounting of the gospel. In this he mentions how Jesus appeared to many after he was raised. The last he appeared to that is mentioned here is Paul. An account of Jesus appearing to Paul is also found in Acts.

'Now get up and stand on your feet. I have appeared to you to appoint you as a servant and as a witness of what you have seen of me and what I will show you.

(Acts 26:16, NIV)

In another instance in Acts, Paul recounts his experience of when Jesus appeared to him. In this recounting found in Acts chapter 26, we find that Jesus commissioned Paul as an apostle.

Paul has seen the risen Christ and has been commissioned by him as an apostle. He is a bondservant and has been purchased, owned and ruled by another. In addition, he is called as an apostle by another. It's not as if he decided on his own or called himself to position of apostle. He has been called and he follows his calling.