John Gill Commentary Acts 18:9

"And the Lord said unto Paul in the night by a vision, Be not afraid, but speak and hold not thy peace:" — Acts 18:9 (ASV)

Then spoke the Lord to Paul in the night by a vision
On a certain night as he was asleep, the Lord Jesus Christ appeared to him in a dream, and spoke after this manner to him:

be not afraid ;
it is very likely that after the baptism of Crispus and his family, and of many of the Corinthians, that both the Jews and the Gentiles were exasperated against the apostle; and his life might seem to be in danger, and he might be thinking of removing from hence for his preservation and safety; and might be advised to it by his friends, or at least that he should be incognito, and not be seen publicly: wherefore the Lord appears to him, and bids him not indulge any fears, or conceal himself and be silent,

but speak, and hold not your peace ;
preach freely and boldly the Gospel without fear of men; the fear of men should not stop the mouths of Christ's ministers.