Paul, an apostle—not from men nor through man, but through Jesus Christ and God the Father, who raised him from the dead— and all the brothers who are with me.
Galatians 1:1-2 (ESV)
Paul's calling as an apostle is intrinsically connected to his relationship with Jesus Christ.
Being an apostle is bearing witness to the gospel's truth. The gospel teaches us that those who are in Christ stand perfect and complete, lacking nothing because of God's abundant grace. God no longer counts our sins against us because of what Christ Jesus accomplished on the cross.
Salvation is fully God's work, not ours.
We cannot do anything to add to our salvation. Everything, from justification (our right standing in Christ) to sanctification (our right life through Christ) and ultimately glorification (our ultimate conformity to Christ), rests on God's perfect and righteous work.
It is important to remember that Jesus commissioned Paul, not anyone else. He was so adamant about proclaiming the gospel because he knew that his life's mission was to reveal God's incredible grace to everyone. Similarly, Jesus has already called every son and daughter of God to be apostolic and prophetic in spreading the gospel of good news over the world. That is why we are to go and declare that God's kingdom has come.
The word, “apostolic” alludes to being sent by God. However, this does not mean that everyone should give up their careers to become cross-cultural missionaries. Some may do so, but many of us continue to reach out to people around us and make disciples as the Lord has commanded us. When we embrace the perspective of being called and sent out into the world, we seek every opportunity to demonstrate God's love and grace to everyone, especially those who have yet to believe and receive the good news of Jesus Christ.