John Calvin Commentary Acts 13:31

"and he was seen for many days of them that came up with him from Galilee to Jerusalem, who are now his witnesses unto the people." — Acts 13:31 (ASV)

After he has stated that Christ came out of the grave, which was guarded by the hired agents of His adversaries, he now adds that Christ appeared to many of the disciples, who bore faithful witness to the people. And he calls them witnesses, either in respect to their office (because they were chosen for this purpose, as we have already stated in the first chapter, (Acts 1:8)), or else simply because they openly and freely professed what they knew concerning Christ.

From this it follows that the matter was openly made known in Jerusalem.

And the proof was not insignificant. For, despite the intimidating power of the enemies—who were ready and determined to resist, and omitted nothing—there were, nevertheless, those who openly affirmed that Christ rose again. These were also individuals who saw this event with their own eyes.

Indeed, if any refutation had been readily available, the scribes would not have overlooked it.