ANSWER: The difficulty is in the word "restoration" or "restitution," the original of which is used nowhere else in the N. T. I think it means the fulfillment of all the predictions respecting Christ; in the Old Testament. When a prediction is made, the prophet commits his veracity to the result, and the fulfillment makes it; good, restoring it to its unquestioned state, as many before that may have doubted the truthfulness of the prediction. In verse 18 Christ's sufferings are declared to be fulfillment of prophecy, and here his stay in heaven is a fulfillment of other predictions relating to the universal spread of the preached Gospel until the totality of the Gentiles — not as individuals, but nations, including the Jews — be brought in. Before such times shall have passed Christ comes not from heaven.
— Steele's Answers p. 147.