John Calvin Commentary Acts 13:43

"Now when the synagogue broke up, many of the Jews and of the devout proselytes followed Paul and Barnabas; who, speaking to them, urged them to continue in the grace of God." — Acts 13:43 (ASV)

Who is speaking. The meaning is uncertain. For it may refer to the Jews and proselytes, suggesting that they exhorted Paul and Barnabas not to lose heart but to stand firmly in the grace of God. Nor did they lack cause; for they saw conflicts prepared for them; and they knew that they therefore needed invincible steadfastness to suffer and withstand the assaults of the opposing faction. Therefore, it would be quite consistent that, burning with a desire to press on, they sought to encourage Paul and Barnabas to persevere.

Alternatively, if you refer it to Paul and Barnabas, the meaning is this: they did not reject those who came, but welcomed them courteously and gently, and confirmed and strengthened them, so that they might continue in the grace they had received.

And this word grace first encompasses the faith of the gospel; secondly, those good things that come to us from it; or, to speak more briefly, the calling into the hope of eternal salvation.