“For he mightily convinced the Jews, and that publickly, shewing by the scriptures that Jesus was Christ.”
King James Version (KJV)
18:26 He spake - Privately; and taught publicly. Probably he returned to live at Alexandria, soon after he had been baptized by John; and so had no opportunity of being fully acquainted with the doctrines of the Gospel, as delivered by Christ and his apostles. And explained to him the way of God more perfectly - He who knows Christ, is able to instruct even those that are mighty in the Scriptures.
18:27 Who greatly helped through grace - It is through grace only that any gift of any one is profitable to another. Them that had believed - Apollos did not plant, but water. This was the peculiar gift which he had received. And he was better able to convince the Jews, than to convert the heathens.
Ac 18:28 For he mightily convinced the Jews. That he was a man of God of great power is shown by Paul's allusions to him. See 1Co 1:12 3:4-6.