“For you shall be a witness for him to all men of what you have seen and heard.”
And the reason for this was that he might be a witness for Him to all men of what he had seen and heard, that is of the life, sacrificial death, resurrection and enthronement of Jesus Christ as Lord and Messiah. There is a hint in his use of ‘all men' of what is to come. But it could be interpreted here by his listeners as meaning all Jews of every class.
So Paul had been fully dedicated to God from birth, he had been taught by the greatest teacher in the land, he had been humbled by the glory of the Lord, he had heard the voice of the Lord, he had seen the resurrected Lord, he would receive visions in a trance, his experience had been confirmed by a pious and revered Jew, what more evidence did they need? And it had pointed him to the Lord Jesus Christ, to be baptised in His name.