John Calvin Commentary Acts 19:8

"And he entered into the synagogue, and spake boldly for the space of three months, reasoning and persuading [as to] the things concerning the kingdom of God." — Acts 19:8 (ASV)

Going into the synagogue. From this we gather that Paul began with the company of the godly, who had already given their names to Christ; and secondly, that he came into the synagogue so that he might gather into one body of the Church the rest of the Jews who did not yet know Christ, or at least who had not yet received him.

And Luke says that Paul behaved boldly, so that we may know that his being heard for three months was not due to his craftily concealing the doctrine of the gospel, or his insinuating himself by some obscure and devious methods. Luke also further points to a sign of this boldness, showing that Paul disputed and persuaded concerning the kingdom of God.

And we know that this term often signifies the restoration that was promised to the fathers, and was to be fulfilled by the coming of Christ. For since without Christ there is a dire and confused dispersion and ruin of all things, the prophets, not in vain, attributed to the Messiah who was to come that he would establish the kingdom of God in the world.

And now, because this kingdom brings us back from falling and backsliding to the obedience of God, and makes us sons from being enemies, it consists—

1. In the free forgiveness of sins, by which God reconciles us to himself and adopts us as his people;
2. In newness of life, by which he fashions us and makes us like his own image.

Luke says that Paul disputed and persuaded, meaning that Paul disputed in such a way that he proved with sound reasons what he alleged. Once that was done, he used the goads of godly exhortations, by which he urged his hearers forward. For no profound disputations will make us obedient to God, unless we are moved by godly admonitions.