For thou shalt be his witness to all men, of those things which thou hast seen and heard.

For you will be a witness for him to all men of what you have seen and of what has come to your ears.

for you will be a witness for him to all the world of what you have just seen and heard.

For thou shalt be his witness - As an apostle - to testify to all people that the Messiah has come, that he has died, that he has risen, and that he is the Saviour of the world.
Of what thou hast seen and heard - Of the remarkable proof which has been furnished you of the divine mission and character of the Lord Jesus.

Thou shalt be his witness unto all - Thou shalt proclaim Christ crucified, both to Jews and Gentiles.

For thou shalt be his witness unto all men,.... Gentiles as well as Jews, an eye and an ear witness to them;
of what thou hast seen and heard; as that he saw him personally and alive, and so could witness to the truth of his resurrection; for after he had been seen by all the apostles, he was last of all seen of Paul; and also, that he heard him and received from him the Gospel, and a mission and commission to preach it; for what he preached he did not receive of man, nor was he taught it by any, but he had it by revelation from Jesus Christ.