Paul, Silas and Timothy, in early 50 A.D., visit several Galatian cities evangelized during the apostle's first journey to the area (Acts 13 - 14, 16:4 - 5). Passing through Phrygia, a region shared by Galatia and Asia, the apostle itches to preach in brand new places. His zeal to proclaim God's truth directs his attention to the highly populated areas of western Asia Minor.
" . . . since the Apostle usually turned his steps towards the large towns, where many Jews were established, it is most likely that Ephesus, Smyrna, or Pergamus (Pergamos) was the point at which he aimed . . ." (Life and Epistles of St. Paul, chapter 8).
Paul's will is overridden by God, who allows him to travel through Asia but not preach.
And after passing through Phrygia and the region of Galatia (for they had been forbidden by the Holy Spirit to speak the word in Asia) . . . (Acts 16:6, HBFV throughout).
The team, after the apostle's plan is rejected, heads northwest through the province of Asia. Paul then attempts, when he arrives at the outskirts of Mysia, to go north into Bithynia. His will, yet again, is overridden by the Lord.
They came down to Mysia and attempted to go to Bithynia; but the Spirit did not permit them to go there (Acts 16:7).
Baffled regarding what to do next, the men travel west to the Aegean Sea port town of Troas. After meeting up with Luke (Acts 16:10) Paul receives a vision of a Macedonian man pleading for help (verse 9). Now assured of God's will, the team journeys to Neapolis on the European continent (verse 10).
Why Not Asia?
The Bible offers a few clues regarding possible reasons behind God forbidding Paul to preach in Asia and Bithynia.
The first Macedonian locations the apostle evangelized were Philippi (Acts 16:12 - 40), then Thessalonica (17:1 - 10) and Berea (verses 10 - 12). The churches established in these cities will ultimately be composed of the poorest of the poor.
Now, brethren, we wish to make known to you the grace of God that has been given to the churches in Macedonia, that in a great trial of affliction, their abundant joy and their DEEP POVERTY . . .(2Corinthians 8:1 - 2).
The Lord may have mercifully opted for Macedonia to receive the gospel before wealthier cities like Ephesus out of his love toward those who need the Gospel the most (Acts 16:9, see also Matthew 11:5, Luke 4:18, James 2:5).
Why Not Bithynia?
Paul may have been forbidden from entering the Roman province of Bithynia - Pontus because his efforts were not needed!
The Apostle Peter may have already evangelizing Bithynia before Paul's second journey. He addressed his first epistle to those in Pontus, Bithynia and nearby provinces who were both Jewish and Gentile converts (1Peter 1:1, 14, 2:10, 4:3). His letter reveals a familiarity with the people and their trials born out of personal interactions with them. He even encourages them in the midst of "the fiery trial among you which is taking place to test you" (1Peter 4:12), which testifies of his intimate knowledge of their circumstances.
Evangelism Goals
One of Paul's evangelistic goals was to preach Christ to those who never heard God's truth.
And indeed, I have aspired to preach the gospel of Christ where the name of Christ was not known, so that I might not build on another’s foundation (Romans 15:20).
The apostle wanted to be the first, in a particular area, to lay a foundation of New Covenant truth so that others could later add to it (1Corinthians 3:10). He did not want to build on or take credit for another man's efforts (2Corinthians 10:14 - 16). God's redirection of his preaching fulfilled these goals.
Conclusion
Paul would never again attempt to enter or preach in Bithynia even though he could have tried to do so during his next evangelism tour.
Paul's ban from preaching in Asia, however, was only temporary. God, during the latter part of his second journey, did not redirect him but rather allowed him to preach in Ephesus for a short time (Acts 18:18 - 21). He later returned to the city and served in it for more than three years (Acts 19:1 - 20)!