The mystery of His will

[Jope]

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).

The apostle addresses his brethren in Christ as "partakers of the heavenly calling" (Heb. 3:1). This calling, in another scripture, is styled "the high calling of God in Christ Jesus" (Phil. 3:14). And again, it is spoken of as the calling of the Father of glory (Eph. 1:17, 18). In those who are the subjects of it God is to show in the ages to come the exceeding riches of His grace (Eph. 2:17); and in them also the Lord is to be chiefly admired in the day of the presence of His power, though that is to be a day in which all His works shall praise Him, a day of clouds of witnesses to His glory both in heaven and earth (2 Thess. 1:10, 11).

This participation of the heavenly calling, thus bestowed on the saints, was not made known in other ages as it is now revealed. For it is only to the church that God has abounded in "all wisdom and prudence;" unto the saints only it is that "he hath made known the mystery of his will, according to his good pleasure, which he hath purposed in himself" (Eph. 1:8, 9). In a wondrous manner it is for them to testify, "We have the mind of Christ." His deep things God has revealed to them by His Spirit (1 Cor. 2:10). The mystery of God [and of the Father, and of Christ]. it is for them, with full assurance of understanding, to acknowledge (Col. 2:2). And their title to all this high endowment stands in this — the Son is their prophet. They have been spoken to by the Son, who is "the brightness of God's glory, and the express image of his person;" and "All things," says the Son, "that I have heard of my Father I have made known unto you" (John 15:15). Israel never stood in such privilege as this, God, in sundry measures and in diverse manners, spake to them by His prophets; but their prophets were not the Son, they came not from the bosom of the Father. They were of the earth, and spake of the earth (John 3:31); for Israel were God's earthly people, having their citizenship and their place here. But the saints, or the church,* are the heavenly family, and their Prophet is therefore He who has come from heaven, and testified what He has seen and heard there. He who was "full of truth" dwelt among us — the Son from the bosom declared the Father, and gave us an understanding to know Him (1 John 5:20). In Him and by Him the blessed God is revealed, for we get "the light of the knowledge of the glory of God in the face of Jesus Christ" (2 Cor. 4:6).

Bellett, J. (n.d.). Musings on Scripture. Vol. 1, The Heavenly Calling Foreshown. Retrieved from http://www.stempublishing.com/authors/bellett/CALLINGH.html​