Thank you for the question of Paul’s main activity as a missionary, “As a missionary, what was the main activity Paul did?”
The Book of Acts reveals the purpose and mission of the Apostle Paul. He was called to preached the Gospel of salvation to both Gentiles and Jews.
Acts 9 says this, ““Lord,” Ananias answered, “I have heard many reports about this man and all the harm he has done to your holy people in Jerusalem. 14 And he has come here with authority from the chief priests to arrest all who call on your name.” 15 But the Lord said to Ananias, “Go! This man is my chosen instrument to proclaim my name to the Gentiles and their kings and to the people of Israel. 16 I will show him how much he must suffer for my name.” (A searchable online Bible in over 150 versions and 50 languages). Bible citations are in the New International Version).
Acts 26 in Paul’s testimony before King Agrippa, he says this, ““On one of these journeys I was going to Damascus with the authority and commission of the chief priests. 13 About noon, King Agrippa, as I was on the road, I saw a light from heaven, brighter than the sun, blazing around me and my companions. 14 We all fell to the ground, and I heard a voice saying to me in Aramaic,[a] ‘Saul, Saul, why do you persecute me? It is hard for you to kick against the goads.’ 15 “Then I asked, ‘Who are you, Lord?’ “ ‘I am Jesus, whom you are persecuting,’ the Lord replied. 16 ‘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 and will see of me. 17 I will rescue you from your own people and from the Gentiles. I am sending you to them 18 to open their eyes and turn them from darkness to light, and from the power of Satan to God, so that they may receive forgiveness of sins and a place among those who are sanctified by faith in me.’” (NIV).
Romans 1 says this, “I am obligated both to Greeks and non-Greeks, both to the wise and the foolish. 15 That is why I am so eager to preach the gospel also to you who are in Rome. 16 For I am not ashamed of the gospel, because it is the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile. 17 For in the gospel the righteousness of God is revealed—a righteousness that is by faith from first to last, just as it is written: “The righteous will live by faith.” (NIV).
The founding of churches through out Asia Minor and into Macedonia was a result of his preaching the Gospel. Without the preaching of the Gospel, churches would not have been founded. It was Paul’s desire that those churches would continue to proclaim the Gospel and help the believers to become mature followers of Jesus.
SUMMARY: The activity of the Apostle Paul was to preach the Gospel of salvation to both Gentiles and Jews wherever the Lord sent him.
For more perspectives:
https://www.quora.com/As-a-missionary-what-was-the-main-activity-Paul-did