John Calvin Commentary Acts 18:7

"And he departed thence, and went into the house of a certain man named Titus Justus, one that worshipped God, whose house joined hard to the synagogue." — Acts 18:7 (ASV)

Departing from there. Paul did not change his lodging, which he had with Priscilla and Aquila, because he was weary of their company, but so that he might more closely connect with and gain favor among the Gentiles. For I suspect that this Justus, whom Luke mentions, was a Gentile rather than a Jew.

Nor does the proximity of the synagogue hinder this at all, for the Jews were scattered, so they had no specific area of the city in which to live. Indeed, it seems that Paul chose the house that adjoined the synagogue so that he might further provoke the Jews.

The title and commendation ascribed to Justus confirm this opinion, for it is said that he was a worshipper of God. Although the Jews did not have sincere religion, yet because they all professed the worship of God, it might have seemed that godliness was common throughout the entire nation. But because it was uncommon among the Gentiles to worship God, if any approached true godliness, he is given this unique testimony, which stands in contrast to idolatry.

I also think that the Corinthians, whom Luke speaks of shortly after, were Gentiles. Nevertheless, so that we do not think that Paul’s labor among the Jews was entirely fruitless, Luke lists two of them who believed: Crispus and Sosthenes. Paul himself speaks of them in the first chapter of the First Epistle to the Corinthians (1 Corinthians 1:1, 14). For in his salutation, he makes Sosthenes his fellow in office, after he says that he baptized Crispus. I understand that Crispus is called the ruler of the synagogue, not as if he alone ruled and held authority—because Sosthenes is given the same title shortly after—but because he was one of the leading men.