John Calvin on Acts 10:39-40

39. And we be witnesses. That he may make his words to be believed, he saith, that he and his fellows in office saw with their eyes all those things which they spake concerning Christ. “Ita loqui de rebus probe compertis,” and so spake of things well ascertained, omitted. Shortly after, he taketh the word witness in another sense, when he saith that they were appointed witnesses “Certos testes,” sure witnesses. by God, “Quo significat publicam apostolis impositam esse personam, utque ad hoc peculiariter delectos esse,” by which he intimates that a public character was assigned to the apostles, and that for this they were specially chosen, omitted. and that they are brought forth, as it were, by God, that they might by their preaching bring men unto the faith of Christ. So Paul, in the fifteenth of the First to the Corinthians, (1 Corinthians 15:15,) saith,

“We (saith he) should be found false witnesses [of Gods] unless Christ were risen from the dead.”

And we have already heard by the mouth of Christ, “Ye shall be my witnesses in Judea, Samaria, and at Jerusalem.” And now Peter calleth himself only an historical witness, because he beheld the things which were done. And here he toucheth his death briefly, because it was openly known; he standeth longer upon his resurrection, which was more doubtful, and the knowledge whereof was more available unto faith.