John Calvin Commentary Acts 18:28

"for he powerfully confuted the Jews, [and that] publicly, showing by the scriptures that Jesus was the Christ." — Acts 18:28 (ASV)

He overcame the Jews. By this it appears for what purpose the ability Apollos had (in that he was mighty in the Holy Scriptures) served; namely, because he had strong and forcible proof to reprove and overcome his enemies with it. Also, the subject of the debate is briefly stated: that Jesus is Christ.

For it was undisputed among the Jews that Christ was promised to be the deliverer; but it was a difficult matter to persuade them that Jesus, the Son of Mary, was this Christ, through whom salvation was offered. Therefore, it was necessary for Apollos to dispute concerning the office of Christ in such a way that he might prove that the testimonies of the Scripture were fulfilled in the Son of Mary, and that he might thereby conclude that He was Christ.

Furthermore, this passage testifies that the Scripture is profitable not only to teach but also to break the obstinacy of those who do not willingly obey and follow. For our faith would not otherwise be firm enough unless there were an evident demonstration existing there of those things which are necessary to be known for salvation. Surely, if the law and the prophets had such great light that Apollos thereby manifestly proved that Jesus is Christ, as if he pointed out the matter with his finger, the addition of the gospel must at least bring it about that the perfect knowledge of Christ may be sought from the whole Scripture.

Therefore, it is detestable blasphemy against God when the Papists say that the Scripture is dark and doubtful. For to what purpose should God have spoken, unless plain and invincible truth should show itself in His words? And when they infer that we must stand by the authority of the Church, and that they are not to dispute with heretics from the Scriptures; their objection is sufficiently refuted by Luke. For, since there was nothing more stubborn than the Jews, we need not fear that those weapons upon which Apollos trusted, and by which he overcame them, will not be sufficient for us against all heretics, since by them we gain the victory over the devil, the prince of all errors.