3:21 Till the times of the restitution of all things - The apostle here comprises at once the whole course of the times of the New Testament, between our Lord's ascension and his coming in glory. The most eminent of these are the apostolic age, and that of the spotless Church, which will consist of all the Jews and Gentiles united, after all persecutions and apostacies are at an end.

Acts 3:21 Parallel Commentaries

Apostolic Exhortation
It is noteworthy that Peter, in addressing this crowd, came at once to the very essence and bowels of his message. He did not beat the bush; he did not shoot his arrow far afield, but he hit the very centre of the target. He preached not merely the gospel of good news, but Christ, the person of Christ; Christ crucified--crucified by them, Christ risen, Christ glorified of his Father. Depend upon it, this is the very strength of the Christian ministry, when it is saturated with the name and person …
Charles Haddon Spurgeon—Spurgeon's Sermons Volume 14: 1868

Gladden -- the Prince of Life
Washington Gladden, Congregational divine, was born at Pottsgrove, Pa., in 1836. After graduating at Williams College he was ordained pastor, and occupied pulpits in Brooklyn, Morrisania, N.Y., and Springfield, Mass., until 1882, when he assumed charge of the First Congregational Church of Columbus, Ohio. He has also occupied editorial positions, and has published many books on social and civil reform and the practical application of Christian truth to popular and common life. His style, whether …
Grenville Kleiser—The world's great sermons, Volume 8

That the Christian Miracles are not Recited, or Appealed To, by Early Christian Writers Themselves So Fully or Frequently as Might have Been Expected.
I shall consider this objection, first, as it applies to the letters of the apostles preserved in the New Testament; and secondly, as it applies to the remaining writings of other early Christians. The epistles of the apostles are either hortatory or argumentative. So far as they were occupied in delivering lessons of duty, rules of public order, admonitions against certain prevailing corruptions, against vice, or any particular species of it, or in fortifying and encouraging the constancy of the …
William Paley—Evidences of Christianity