John Calvin Commentary Acts 10:39

"And we are witnesses of all things which he did both in the country of the Jews, and in Jerusalem; whom also they slew, hanging him on a tree." — Acts 10:39 (ASV)

And we are witnesses. So that his words may be believed, he says that he and his fellows in office saw with their own eyes everything that they spoke concerning Christ. Shortly after, he takes the word 'witness' in another sense, when he says that they were appointed witnesses by God and are brought forth by God, as it were, so that by their preaching they might bring people to the faith of Christ. So Paul, in 1 Corinthians 15:15, says:

We (he says) should be found false witnesses [of God] unless Christ were risen from the dead.

And we have already heard from the mouth of Christ, You shall be my witnesses in Judea, Samaria, and at Jerusalem. And now Peter calls himself only a historical witness, because he beheld the things that were done. And here he touches on his death briefly, because it was openly known; he dwells longer on his resurrection, which was more doubtful, and the knowledge of which was more conducive to faith.