John Chrysostom

"For thou shall be His witness unto all men of what thou hast seen and heard." "And hear the voice of His mouth." See how high he raises the fact! "For thou shall be His witness - for this, because thou wilt not betray the sight and hearing - both of what thou hast seen, and of what thou hast heard:" by means of both the senses he claims his faithfulness - "to all men." For he did become a witness to Him, and a witness as it ought to be; by what he suffered, by what he did, and by what he said. …

Homily on Acts 47 ↗

Theophylact of Ohrid

Because you will not betray nor forget what you have seen and heard. "What you have seen and heard." The Lord persuades Saul by means of both senses, that is, by means of sight and hearing. "Because you will be His witness before all people." Not before fellow countrymen only, but also before foreigners, because witnesses are witnesses precisely in order to testify not before those who know, but before those who do not know. …

Commentary on Acts ↗