Adam Clarke on Acts 18:9

Then spake the Lord to Paul in the night by a vision - It is likely that Paul was at this time much discouraged by the violent opposition of the Jews, and probably was in danger of his life; see Act 18:10; and might have been entertaining serious thoughts of ceasing to preach, or leaving Corinth. To prevent this, and comfort him, God was pleased to give him this vision.

Be not afraid - That this comfort and assurance were necessary himself shows us in his first epistle to these Corinthians, Act 2:3 : I was with you in weakness, and in fear, and in much trembling.