They went through the region of Phrygia and Galatia, having been forbidden by the Holy Spirit to speak the word in Asia.  When they had come opposite Mysia, they attempted to go into Bithynia, but the Spirit of Jesus did not allow them; so, passing by Mysia, they went down to Troas.  During the night Paul had a vision: there stood a man of Macedonia pleading with him and saying, "Come over to Macedonia and help us."  When he had seen the vision, we immediately tried to cross over to Macedonia, being convinced that God had called us to proclaim the good news to them.

Not once but twice the Holy Spirit prevented Paul and the disciples with him from preaching the gospel in certain regions and cities. This is after receiving Christ's command to "go into all the world and proclaim the good news to the whole creation." (Mark 16.15)  Further we read again and again how powerfully the Holy Spirit was with them.  Despite that fact they still had to go through a discernment process to decide where to preach next.  Sometimes their conclusions were not God's will so they continued to move on.  In short they did not become discouraged nor fail in their missionary activity by the apparent contradiction of being commissioned by God to preach the gospel to the whole world yet prevented by him from preaching in certain regions.  They did not become paralyzed with indecision or confusion of their mission they simply chose another region and moved on.

It wasn't always the Holy Spirit who prevented them from preaching.  When they wanted dearly to visit the Thessalonians Satan blocked their way. (1 Thessalonians 2.18)  We know no more than that but clearly they possessed the Spirit of discernment to know when it was the Holy Spirit who prevented their advancement and when it was Satan.

There are many deep lessons to learn from this passage as we look at our own lives, which are often seemingly confused and random.  Sometimes we firmly believe we're following God's will in a certain decision, even when it is accompanied by much prayer.  Yet in the end it doesn't work out.  We may blame ourselves and distrust our discernment.  The confusion may even paralyze us for many years if not the rest of our lives.  But if we continue to battle to do God's will, continue to move forward and do not grow weary and stop, we will eventually see, usually in hindsight, that God's will does unfold in our lives in the end.  It is usually never what we expect.