My opinion (I think I can defend it from Scripture) is that when Paul made it to Corinth, he was tired. At this point his mission trip had been going on for a year. I think Paul was lonely – he had been alone since he was in Berea; he was alone in Athens ministering for some time; he was alone on his 50-mile journey to Corinth. I think Paul was discouraged, knowing that the pattern of his ministry would likely be repeated again in Corinth: preaching the gospel to the Jews, rejected by the Jews, arrested or mobbed, maybe beaten or imprisoned, and forced to sneak out of another town in the dark of night.
I believe that for two Scriptural reasons. First, some time later Paul wrote two letters to the believers of Corinth, and when he reflected on his state of mind when he first arrived there, he wrote, “I was with you in weakness, in fear, and in much trembling” (1 Corinthians 2:3). Maybe Paul was even thinking about moving on, quitting. Then in Acts 18:9-10, Jesus (in His infinite wisdom) appeared to Paul in the night and said some things that we can only assume Paul desperately needed to hear.
Jesus encouraged Paul about the city of Corinth (v.10). Outside of Rome, Corinth was the most important city in the Roman Empire. With two major ports, Corinth was perfectly situated to serve as the hub for a church planting movement that would make a difference in that part of the world!
Jesus also encouraged Paul by bringing some partners into his life. Aquila and Priscilla (v.2) came along and shared their lives with Paul. They were good partners, and God brought them into Paul’s life at just the right time. Silas and Timothy re-joined him in Corinth — the mission team was together again, and none too soon (v.5). After a very dramatic break from the Corinthian Jewish community (v.6), a man named Justus opened his home for Paul to use as a place for ministry. (Thank God for people who open their homes and their schedules for the Lord to use for Bible studies, Sunday School fellowships, and people who need ministry.) And besides Justus, Priscilla and Aquila, Silas and Timothy, verse 8 says there were many Corinthians hearing the gospel, believing, and being baptized. God was forming a community of believers (a church), and that was a great encouragement to Paul.
Furthermore, Jesus gave Paul those amazing promises in verses 9-10. Jesus promised His presence: “Do not be afraid, I am with you.” Jesus promised His protection: “No one will attack you to harm you.” And Jesus promised His people: “I have many in this city who are My people” – people who had believed (or would believe) and were going to stand with Paul in the work of the gospel. That encouraged Paul to stay on for 18 months! Luke only gives us one example of God’s protection in Corinth, but it was a major one (v.12-17). If God used the calloused judgment of a Roman judge to protect Paul, just think of all the ways He can protect and provide for you! Trust Him!