When Jesus told Paul (in a vision) to stay in Corinth despite rising Jewish opposition, the Lord gave three reasons to support His command:
(1) “For I am with you” (Acts 18:10a)
(2) “No one is going to attack and harm you” (Acts 18:10b)
(3) “I have many people in this city” (Acts 18:10c)
While the first promise is a familiar reassurance of divine presence and the second promise is a unique promise to Paul applicable only in Corinth, the third promise is strange because of the seeming ambiguity of who the “many people” are.
I see only two possible interpretations of the “many people” here:
(1) The “many people” are Christians
(2) The “many people” are not yet Christians–but will be
I don’t think the “many people” can be those who have converted already. This third promise of v. 10 is given as a reason for the second promise (“no one is going to attack and harm you, because I have many people in this city”), and it doesn’t make much sense to promise Paul that he won’t be persecuted because there are already Christians in Corinth.
After all, Paul was persecuted in Philippi (Acts 16:22-24) after believers were converted (Acts 16:14-15). There seems to be no reason in Acts that Paul shouldn’t worry about persecution simply because of the presence of other believers.
However, there is reason to think that Paul shouldn’t worry about extreme opposition (namely, martyrdom) if the Lord promised that people will still believe under his ministry. In other words, if people will still believe under his ministry, it’s because he is still around to have a ministry! Any opposition he faces in Corinth, then, will not physically harm and destroy him.
So I think it’s best to interpret “many people” (Acts 18:10c) as those who have not yet believed but who will believe under Paul’s preaching.
But what about the first part of the third promise found in Acts 18:10c? Jesus said, “I have many people in this city [Corinth].” What would such a possessive statement indicate? The people aren’t yet saved, but the people are in some sense…His.
Could Jesus’ possessive words be akin to John 6:37, “All that the Father gives me will come to me, and whoever comes to me I will never drive away”? The “many people” in Acts 18:10 would be those who the Father has given to Jesus and who will come to him in faith.
Jesus’ words in Acts 18:10 probably sound the same notes of predestination as Acts 13:48: “…and all who were appointed for eternal life believed.”
In summary, the appointment preceded the believing (Acts 13:48), being given to Jesus preceded coming to him in faith (John 6:37), and–in the passage in question–Jesus has “many people” before they are ever converted (Acts 18:10).
In Acts 18:10, Jesus told Paul to remain in Corinth and keep preaching, because He had “many people” there. These “people” had not yet been converted, yet they were already His by virtue of being appointed to eternal life. Jesus is saying, “Paul, stay in Corinth a while longer, because there are many elect ones of mine who have not yet believed–and who will believe under your ministry.”
So Paul stayed (Acts 18:11).