The great missionary of the earliest church was Paul of Tarsus. After his conversion to the Christian faith, he brought the gospel throughout Asia Minor (Turkey), to Greece, and ultimately to Rome.

Saul of Tarsus
Saul was a Roman citizen from birth. He was born in Tarsus in Cilicia and apparently had a thorough education in Hellenistic learning. At the same time, he was a Jew and could trace his ancestry to the tribe of Benjamin. He received his religious training in Jerusalem, under the famous teacher Gamaliel. Saul identified himself as a Pharisees, with high regard for the Law of God and expecting the coming of the Messiah.
We first meet Saul in Acts 8:1, assisting in the execution of Stephen. The zealous young man took the initiative to seek out other Christians and arrest them. He even tracks down Christians in the Jewish community in Damascus, Syria.
But on the way to Damascus, he meets Jesus Christ in a blinding vision. This experience turns Saul fully around. The most zealous persecutor of Christians becomes the most zealous preacher of Christ.
Paul’s commission
Acts 9:15 identifies Saul of Tarsus as “a chosen instrument of God to carry his name before the Gentiles”. Because of his past, the new apostle cannot function well among the Jewish-Christian community. Instead he becomes a missionary preacher to the Gentiles (non-Jews). The book of Acts details three of his journeys through Asia (= Turkey), Macedonia, and Greece. For this work, he adopts the Latin name “Paul(us)”.
After his conversion, Paul spent a few years in Arabia and Syria. Then me met with the apostles in Jerusalem, who recognized his calling as an apostle. (It is therefore not justified to pitch Paul and his teaching against James and Peter, as some have done!) Paul returned to his homeland Cilicia, until Barnabas asked him to come to help out in the new church in Antioch. After a few years, this church commissioned Paul and Barnabas as missionaries.
Paul’s own understanding of his work is clear from his sermons and epistles. “For so the Lord has commanded us, saying, ‘I have made you a light for the Gentiles, that you may bring salvation to the ends of the earth.’” (Acts 13:41, quoting Is 49:6) Especially telling is the beginning of his letter to the Romans:
Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God […] concerning his Son, […] Jesus Christ our Lord, through whom we have received grace and apostleship to bring about the obedience of faith for the sake of his name among all the nations […] (Rom 1:1-5).
The Jerusalem Council
When Peter baptized the Roman officer Cornelius, it raised some eyebrows. Now that Paul and Barnabas are actively making converts among the Gentiles, stronger opposition emerges. The so-called Judaizers insisted that new converts to Christianity should first become Jews. Specifically, they must observe the Jewish dietary laws, keep the Sabbath, and become circumcised.
The church leaders deal with this question in a meeting, called the Jerusalem Council. Acts 15 gives a detailed report. The council concludes that the conversion of the Gentiles is fulfillment of ancient prophecy, and that these new believers should not be harassed. They must, of course, remove from the lives all idolatry and immorality; but they do not need to adopt Jewish identity.
This decision does not immediately put an end to the Judaizing movement. Paul frequently deals with the problem, e.g. in his epistle to the Galatians. But in the end, the Jerusalem Council carries the day: the Christian church distances itself from specific Jewish customs, declaring that Jesus as the fulfillment of the Law also puts an end to the ceremonies of that Law.
Paul’s preaching in the Hellenistic world
Although Paul is a zealous proponent of the Jerusalem council, he has not forgotten about the Jews. In every town he visits, he would first preach in the Jewish synagogue. As a Hellenistic Jew, he has an “in” with this community. When the Jews reject his message, Paul would continue his work outside of the synagogue.
While Paul reaches many “Gentiles”, it is likely that most of these new converts were “god-fearers”, already interested in the Jewish religion; see e.g. Acts 13:16.
We meet a very Jewish Paul, who conducted his mission almost entirely within the bounds of the synagogue and the circle of God-fearing Gentiles attached to it. (Skarsaune, In the Shadow of the Temple, p. 174)
Because of the location of Jewish communities and synagogues, Paul’s worked mostly in larger towns and cities. His stay was often short because of rising persecution. Paul often revisited churches he had planted on later trips, sent helpers, or wrote letters.
Paul and paganism
Acts reports only a few cases of Paul preaching to true pagans. Most famous is his speech in Athens (Acts 17). In it, he tries to find common ground with pagan worship and philosophy; however, he boldly preaches the resurrection of Jesus, which for most Athenians is a deal-breaker.

Paul also has run-ins with Hellenistic paganism. In Philippi, he and Silas heal a demon-possessed girl who was exploited as a fortune-teller. This brings them into jail; the jailer becomes a believer. (Acts 16:16ff)
In Ephesus, employees of the Artemis temple riot because they view Paul’s preaching as a threat to their livelihood and their city. It takes a visit from the town clerk to quiet them down. (Acts 19:21ff)
All in all, Paul’s message and method fall short of the sophisticated standards of Greek culture. The message of the gospel is “foolishness” to the Greeks (1 Cor 1). Paul relies on God’s wisdom rather than that of the world, on God’s power rather than human persuasion.