How did Apollos prove to the Jews that Jesus is the Messiah using the scriptures?

In Acts 18:28 it says "For he vigorously refuted his Jewish opponents in public debate, proving from the Scriptures that Jesus was the Messiah", back then the new testment hadn't been written yet. Additionally, it says Apollos himself didn't have prior knowledge of the close miracles of Jesus or about the Holy Spirit, as those were only explained to him later, after he met Priscila and Aquila. So how did he do that? Is it possible only with the old testment?