Here is the theme of the early preaching of Acts: testes sumus!
Be a witness!
"A man is not primarily a witness against something, That is only incidental to the fact that he is a witness for something. A witness, in the sense that am using the word, is a man whose life and faith are so completely one that when the challenge comes to step out and testify for his faith, he does so, disregarding all risks, accepting all consequences."
Witness, Whittaker Chambers, p. 5