18:6 But when they opposed him (Paul) and blasphemed, he shook his garments and said to them, “Your blood be upon your own heads; I am clean. From now on I will go to the Gentiles.” 7 And he departed from there and entered the house of a certain man named Justus, one who worshiped God, whose house was next door to the synagogue. 8 Then Crispus, the ruler of the synagogue, believed on the Lord with all his household. And many of the Corinthians, hearing, believed and were baptized.
9 Now the Lord spoke to Paul in the night by a vision, “Do not be afraid, but speak, and do not keep silent; 10 for I am with you, and no one will attack you to hurt you; for I have many people in this city.” 11 And he continued there a year and six months, teaching the word of God among them.
Visions and direct messages from God are not common. They never have been. His desire for us is to walk by faith, not by sight. When visions and messages come it is generally for one of four reasons. Usually it's because things are really bad and God gives a man a message to guide the people back to where they should be. The Bible is full of such examples. Another reason God gives direct messages is when specific guidance is needed like when Paul and his companions were given a message to "come to Macedonia". A third reason God sends direct messages is to reveal future events as in the case of Joseph telling his family they would bow to him some day or God revealing the future to Daniel. This is done so we can look back and see that God knew all along what was going to happen and we can trust Him.
The fourth reason God speaks directly to us is when it is desperately needed. When an individual who is trying to follow Him is so low and discouraged that God sends a message to pick them up and encourage them. In the Old Testament Elijah was such a case. For years he lived a fugitive from his own king. Finally there was a showdown between the king and his idol worship and Elijah and his faith in God on Mount Carmel. Of course God was vindicated and Elijah believed this would change everything. It didn't. The very next day he was sent a message that the queen was going to have him killed. He was so discouraged over the whole mess that he became suicidal and fled 250 miles into the desert. God met him there with a message just for him to encourage him to keep going.
Here Paul is for the time a tired and almost broken man. He has preached from city to city. He has been beaten and imprisoned and handled it all with grace and faith. Now in Corinth all it took was some resistance from the Jews and he became unusually angry and frustrated.
He was human. He was tired. He was emotionally spent. God knew. God understood. God met him in his discouragement. He basically said "Paul I know you're frustrated and tired but Corinth is going to be good for you. I will protect you here. Just keep preaching boldly and watch what happens. Many here are ready to listen and follow Me."
Paul was able to stay peacefully for 18 months. We believe the longest he stayed in one place for quite some time. He needed the stability and the peace and God brought him to a place where he could have it. Why? Because He is the Great Shepherd. He knows His sheep. We can trust Him.