“Him God raised up the third day, and gave him to be made openly known, not to all the people, but to witnesses who were chosen beforehand of God, even to us, who ate and drank with him after he rose from the dead.”
‘Him God raised up the third day.' Compare 1 Corinthians 15:3-4. The fact that God raised Him within three days revealed that God did not see Him as deserving of death. Rather it demonstrated that He was God's favoured One, God's Messiah, and that His death must therefore have been for us. Having been raised within three days death had never mastered Him.
So as in his previous speeches he again stresses the resurrection, and again points to those who are witnesses, thus making a twofold emphasis on witnesses (compare Acts 10:39). He points out that God made the Risen Jesus ‘openly known' to witnesses chosen beforehand in such a way that His resurrection could not be doubted, because He ate and drank with them after rising from the dead. He had not left any doubt on the matter. And Peter had been one of them.