If Jews had accepted Christ as their Messiah would the earthly kingdom have been established then?

The question you asked is one that has been long debated even among dispensationalists and it has been used by those who oppose Dispensationalism. With all such theological questions, we must seek our answers in the Scripture and not by what simply seems logical to us, assuming the Scripture does truly teach this or that doctrine. A case in point is the doctrine of the Trinity which truly defies our own logic or understanding. It’s biblical and we believe it, but who can understand it? So the point is, what does the Bible teach regarding the offer of the kingdom?

If there had been repentance by the nation of Israel, as Joel prophesied and other Old Testament prophets, the Kingdom would have been established. This seems clearly the point of Peter in Acts 3:17f.

3:17 And now, brothers, I know you acted in ignorance, as your rulers did too. 3:18 But the things God foretold long ago through all the prophets, that his Christ would suffer, he has fulfilled in this way. 3:19 Therefore repent and turn back so that your sins may be wiped out, 3:20 so that times of refreshing may come from the presence of the Lord, and that he may send the Messiah appointed for you—that is, Jesus. 3:21 This one heaven must receive until the time all things are restored, which God declared from times long ago through his holy prophets. 3:22 Moses said, ‘The Lord your God will raise up for you a prophet like me from among your brothers. You must obey him in everything he tells you.3:23 Every person who does not obey that prophet will be destroyed and thus removed from the people.’3:24 And all the prophets, from Samuel and those who followed him, have spoken about and announced these days. 3:25 You are the sons of the prophets and of the covenant that God made with your ancestors, saying to Abraham, ‘And in your descendants all the nations of the earth will be blessed.’3:26 God raised up his servant and sent him first to you, to bless you by turning each one of you from your iniquities.”

The “times of refreshing” has generally been acknowledged as a definite reference to the Messianic age or the millennial kingdom. Of this, Ryrie has the following footnote to verse 19: “return. I.e., turn from sin to God by reversing their verdict about Jesus and confessing Him as the Messiah. times of refreshing and restoration of all things (v. 21) refer to the millennial kingdom.”

Also, the Bible Knowledge Commentary has the following comment:

Was Peter saying here that if Israel repented, God’s kingdom would have come to earth? This must be answered in the affirmative for several reasons: (1) The word restore (3:21) is related to the word “restore” in 1:6. In 3:21 it is in its noun form (apokatastaseo„s), and in 1:6 it is a verb (apokathistaneis). Both occurrences anticipate the restoration of the kingdom to Israel (cf. Matt. 17:11; Mark 9:12). (2) The concept of restoration parallels regeneration when it is used of the kingdom (cf. Isa. 65:17; 66:22; Matt. 19:28; Rom. 8:20-22). (3) The purpose clauses are different in Acts 3:19 and 20. In verse 19 a so that translates pros to (some mss. have eis to) with the infinitive. This points to a near purpose. The two occurrences of that in verses 19b and 20 are translations of a different construction (hopo„s with subjunctive verbs), and refer to more remote purposes. Thus repentance would result in forgiveness of sins, the near purpose (v. 19a). Then if Israel as a whole would repent, a second more remote goal, the coming of the kingdom (times of refreshing at the second coming of Christ) would be fulfilled. (4) The sending of the Christ, that is, Messiah (v. 20) meant the coming of the kingdom. (5) The Old Testament “foretold these days” (v. 24; cf. v. 21). The Old Testament prophets did not predict the church; to them it was a mystery (Rom. 16:25; Eph. 3:1-6). But the prophets often spoke of the messianic golden age, that is, the Millennium.

This offer of salvation and of the Millennium pointed both to God’s graciousness and to Israel’s unbelief. On the one hand God was giving the Jews an opportunity to repent after the sign of Christ’s resurrection. They had refused the “pre-Cross” Jesus; now they were being offered a post-Resurrection Messiah. On the other hand Peter’s words underscore Israel’s rejection. They had been given the sign of Jonah but still they refused to believe (cf. Luke 16:31). In a real sense this message confirmed Israel’s unbelief.

Some Bible scholars oppose the view that the kingdom was offered by Peter. They do so on the basis of several objections: (1) Since God knew Israel would reject the offer, it was not a legitimate offer. But it was as genuine as the presentation of the gospel to any nonelect person. (2) This puts kingdom truth in the Church Age. However, church truth is found before the church began at Pentecost (cf. Matt. 16:18; 18:17; John 10:16; 14:20). (3) This view leads to ultradispensationalism. But this is not a necessary consequence if this offer is seen as a transition within the Church Age. Acts must be seen as a hinge book, a transition work bridging the work of Christ on earth with His work through the church on earth.

In conclusion, Acts 3:17-21 shows that Israel’s repentance was to have had two purposes: (1) for individual Israelites there was forgiveness of sins, and (2) for Israel as a nation her Messiah would return to reign.

Many other commentaries and books of prophecy could be quoted to show this is a common view. The point is this, God made a promise to the Nation that on repenting and turning, He would give them their promise through Messiah and set up the kingdom and God will not go back on His word. But this is only part of the story. He also had a plan to reach the nations through Messiah through Israel, and knowing their stubborn and hardness of heart, He chose to use that stubbornness to set Israel aside temporarily to extend the offer of salvation to the nations. This is anticipated in Isaiah 6:9-13 (see also Matt. 13:13f; Romans 9-11).

The cross is in no way God’s “Plan B” for the prophets spoke of both the sufferings and the glories as 1 Pet. 1 shows (as does Acts 3:17-21). If Israel had repented before the cross, Christ would still have been put on the cross by the Romans. But all of this is purely hypothetical. In keeping with God’s promises, Christ had to offer the kingdom, but also, in keeping with God’s eternal plan to reach the world, Israel was bound to reject as God knew they would and the gospel was offered to the world. Read carefully Romans 11:7-28. Again, as Paul states in Romans 11, this is a mystery truth.

Israel’s corporate stumbling, which is temporary, not permanent, is called a mystery. In Scripture a mystery is not a truth difficult to understand, but a truth previously unrevealed (and therefore unknown) which is now revealed and publicly proclaimed (cf. Eph. 3:9; Col. 1:26; at Matt. 13:10-16, see the chart that lists NT mysteries). Paul wanted to make sure his Gentile readers knew about the mystery concerning Israel in God’s sovereign choice. God’s purpose was so that you may not be conceited (lit., “wise in yourselves”). God’s sovereign plan to put Israel aside temporarily in order to show grace to Gentiles is no basis for conceit on the part of the Gentiles; it is designed to display further the glory of God.

Neither the offer of salvation of the Gentiles nor Israel’s hardening was a mystery in the Old Testament, as the verses above show, but the purpose of God in that hardening, was a mystery. All of this goes beyond our comprehension because our intelligence and capacity for comprehension is finite. The difference is even far greater than the difference that exists between a man and his dog. You can train your dog to go get the paper, maybe even your slippers, but you can’t sit down and read it to him and then discuss the news. Some things are just beyond our comprehension. A legitimate offer of the kingdom was made, but in the sovereign will and plan of God, there was no way that Israel as a nation was going to turn and repent. Some did, a remnant, but in God’s plan He is reaching out to the world through the rejection and hardness of Israel’s heart, a partial hardening until sometime in the future, via the awful testings of the Tribulation, when they will be brought to their knees, humbled, and will repent. To say, “What if they had repented” misses the point and is pure human speculation. They didn’t and God knew they would not, but in His faithfulness, He made the offer.