20 and that He may send Jesus Christ, who was preached to you before, 21 whom heaven must receive until the times of restoration of all things, which God has spoken by the mouth of all His holy prophets since the world began.
Should Israel repent of their rejection of their Messiah, God will bring in a time of restoration to the nation. He will do this by sending Christ again at the Second Coming.
and that He may send Jesus Christ [Messiah],
The sending of Christ, the Messiah, will launch the Millennial reign of Christ for 1,000 years. He will be the King of the World.
who was preached to you before,
Peter’s sermon demonstrated that Jesus was the Messiah to his audience of Jews.
whom heaven must receive until
The resurrected Christ must remain in heaven until Israel repents of rejecting their Messiah. The “until” here is the entire time between the Pentecost and the Second Advent. People were confused as to why, if Jesus was the Messiah, He did not set up His kingdom on earth while He was there (Mark 9:12).
the times of restoration of all things,
The “restoration” of the nation Israel is what people in the upper room thought may come after the ascension of Christ (Acts 1:6). The “restoration” is not a reference to the church because the prophets of the Old Testament never predicted the coming of the church. The church was a truth not previously revealed to Israel (Ro 16:25; Eph 3:1-6). The restoration will rectify the effects of the fall of man into sin. The Messiah will reverse the effects of the fall where the lion will feed with the lamb, and surd evil issues on earth will not exist anymore, such as tornados and hurricanes.
which God has spoken by the mouth of all His holy prophets since the world began.
Peter’s message was familiar to Jews because many passages in the Old Testament predicted a significant change in God’s economy for the Jews. This verse and verse 24 speak of Old Testament prophecies about the “restoration of all things,” the messianic age (He 9:10). Peter could appeal to Scripture because Jews had a high view of the Bible.
God’s grace extends beyond flagrant violation.
God’s offer of salvation and the coming kingdom points to God’s grace, even in the face of Israel’s flagrant unbelief. The Father provides opportunity even after the stark rejection of His Son. Israel rejected both the promise and the actuality of the risen Messiah (Lu 16:31).