The mystery of His will

A mystery in scripture is something hitherto unrevealed. In Eph. 1:9 we find that there is a mystery of the will of the Father. This mystery is speaking of the will of God to "show his wrath and to make known his power"; to show great mercy on the "vessels of wrath [the Gentiles] prepared for destruction" (Rom. 9:22, ESV).

Romans 9 ESV, bold emphases mine
18So then he has mercy on whomever he wills, and he hardens whomever he wills.

22What if God, [willing] to show his wrath and to make known his power, has endured with much patience vessels of wrath prepared for destruction, 23in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory- 24even us whom he has called, not from the Jews only but also from the Gentiles?​

Since such a wrath required for the Gentile sinners is more than that of the Jewish sinners (Luke 7:42-43; John 7:48-49), since the Church is predominantly Gentile and since that Church must have her punishment atoned for via the substitute Christ (2 Cor. 5:21), God revealed his mystery will doing just that, "[showing] his wrath and...[making] known his power" (Rom. 9:22, ESV) by punishing Christ on the Cross as a substitute for the Gentile's great(er) sins.

This is why the Kingdom offer had to be rejected by Israel. The mystery of the will of the Father "[showing] his wrath" (Rom. 9:22, ESV) through the cross had to happen.

"And Jesus uttered a loud cry and breathed his last...And when the centurion, who stood facing him, saw that in this way he breathed his last, he said, “Truly this man was the Son of God!”" (Mark 15:37, 39, ESV).​

Paul said that now, when mystery has been revealed, God shows to the angels, by the predominantly Gentile Church, the manifold wisdom of Himself (Eph. 3:10). These same angels rejoice over this revealed mystery of the great grace on the vessels fitted for destruction, as attested to by our Lord and Peter (Luke 15:1-10, especially verse 10; 1 Pet. 1:12).

This atonement was not only for the Church though, who first trusts in Christ (Eph. 1:12). This atonement does extend to the Jews that don't now comprise the Church, for their father's sakes and the covenants made to them (Rom. 11:25-29). The Jews have now been put into the position (of having no God) that the Gentiles occupied in the past. But "if [the predominantly Gentile Church] were cut from what is by nature a wild olive tree, and grafted, contrary to nature, into a cultivated olive tree, how much more will these [the Jews], the natural branches, be grafted back into their own olive tree" (Rom. 11:23-24, ESV).