John Calvin Commentary Acts 18:27

"And when he was minded to pass over into Achaia, the brethren encouraged him, and wrote to the disciples to receive him: and when he was come, he helped them much that had believed through grace;" — Acts 18:27 (ASV)

When he was determined. Luke does not express why Apollos wanted to go to Achaia. Nevertheless, we gather from the context that he was not attracted by any personal gain, but because an opportunity for more abundant fruit in spreading the gospel presented itself there; because the brothers further encouraged him with their exhortation, and spurred him on when he was already running.

They would not have done this, unless it had been for the common benefit of the Church. For it would have been an absurd thing to urge a man to depart to another place, whose diligent service they were already using and knew that they would need him afterward, unless a greater benefit was anticipated.

And I understand that the brothers of Ephesus wrote to those in Achaia, not only that they should provide lodging for him, but also that they should allow him to teach. This is indeed holy commendation, when we endeavor to extol every good person with our testimony and support, lest the gifts of the Holy Spirit, which He has given to everyone for the edifying of the Church, lie buried.

When he came. The brothers foresaw this, who had already experienced it, when they urged him to undertake that journey which he had already conceived in his mind. And when it is said that he greatly helped the faithful, we can understand this in two ways: either that he helped those who were not so well equipped, and supported them in overcoming the pride of their enemies (for not everyone was able to have a weapon ready to undertake a difficult battle against established opponents, who would never have yielded unless they had been compelled); or that he aided them, so that their faith would not fail when shaken by the opposition of their enemies, which often befalls the weak.

I understand that they were helped in both ways: that, by having a skillful and experienced leader, they gained the victory in the conflict; and secondly, that their faith was fortified with new support, so that it might be free from the danger of wavering. Furthermore, Luke seems to note that the brothers were helped by this boldness and steadfastness, when he says that he disputed publicly with the Jews.

For this was a sign of zeal and boldness, not to flee the light. Now, concerning the words through grace used at the end of the sentence, it either agrees with the preceding words, they believed, or else it must be referred to the help with which he helped the brothers. The former interpretation is not difficult, for its meaning would be this: that the faithful were illuminated by the grace of God, so that they might believe; as if he had said that the brothers, who were already called by God's grace to faith, were assisted. Yet the other interpretation seems to fit better: that Apollos, by imparting the grace he had received to the brothers, helped them. So that through grace would mean 'according to the measure of the grace received.'