The mystery which Paul writes about in the above text is a direct reference to the preceding verses, in which the Apostle sketched the symbol of the olive tree, from which 'natural branches' have been cut off, and in their place wild branches were grafted. In this way the Jews who did not accept the faith in Jesus did not enter the New-Covenant-based relationship with God, did not receive the gift of the Holy Spirit, and consequently were not granted the privilege of participation in the Church - the Body of Christ. At the same time the offer of "high calling" was presented to the Gentiles, and the elect who have accepted this invitation and repented, were grafted onto the olive tree as 'wild branches' to replace the 'natural branches' (Rom. 11:16-24). Therefore, many Gentile believers came to the conclusion - and still the majority of Christians believe so - that the nation of Israel was rejected by God, ceased to be of interest to Him in any special way, and even more so, was condemned by God for rejecting Jesus Christ.
"... until the fullness of the gentiles comes in"
Meanwhile, Paul writes that this 'high opinion' Gentile believers have of themselves is not justified by facts. Jews still remain "beloved on account of the fathers; for unrepented of [are] the gifts and the calling of God" (Rom. 11:28,29 YLT). God, who knows the end from the beginning (Is. 46:10), also knew that only a part of Israel would believe in Jesus. Proponents of the thesis that God rejected Israel present it in such a way as if God suddenly became impatient and began to regret His choice of this nation. But God is neither impatient nor does He regret. Little faith in this nation resulted only in God sending the invitation to the Church to the Gentiles, and consequently some positions within 'royal priesthood' will be granted to the elect from amongst the Gentiles, even though originally it was promised to Israel (Mt. 22:1-14; 1 Pet. 2:9; Ex. 19:5,6).
It does not mean in any sense that God has rejected this part of the nation which has not accepted Jesus. The Apostle Paul points out that "all Israel will be saved" (Rom. 11:26 GNB), but in its due time. First, God's instrument of salvation must be prepared, i.e. the 'little flock' of co-kings and co-priests with Jesus Christ (Lk. 12:32; Rev. 5:9,10, 20:4-6). The nation from which members of the 'royal priesthood' were first recruited - which was chosen by its origin - was Israel. In this sense, Abraham is the father of both earthly and heavenly seed of promise (Gen. 22:17). However, while Jews are the descendants of Abraham by flesh, his descendants by spirit are those who keep his faith (Lk. 3:8; Rom. 4:11). Because few Jews showed it, the calling to the Body of Christ (and therefore also hope of obtaining the highest reward of this calling in the 'royal priesthood') has been extended to the Gentiles (Acts 28:25-28).
However, Jews are still 'natural branches'. Even the Apostle Paul, whose special ministry was to preach to the Gentiles, at the outset of his work in a community always first directed his steps towards the local synagogue (Acts 13:46, 22:21, 28:28). The status of Israel as a chosen nation, still widely questioned, will become manifestly evident when 'the fullness of the Gentiles comes in' (Rom. 11:25 KJV). When 'royal priesthood' is collected, then will begin "the times of restitution of all things" expected since the fall in Eden (Acts 3:19-21 KJV). Even though in the Bible we find the promise that "in the name of Jesus every knee may bow' (Phil. 2:10,11), the first nation to be restored to harmony with God will be Israel, "as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob" (Rom. 11:26 KJV, Zech. 12:7).
It is with Israel that the New Covenant will be made, of which we read in Rom. 11:27 and Jer. 31:33,34, and which then will encompass all the nations (Zech. 8:20-23, 14:16-19). It is Israel that will take the place of the first nation, and it is through Israel that God will bless all the subjects of His Kingdom, as was stated in the promise given to Abraham (Gen. 22:18; Zech. 8:13; Is. 2:3). So there is no question of Israel's rejection - all Israel will not only be saved, but it also will take a leading role in managing the earthly phase of the Kingdom, thus fulfilling all the provisions of the covenant which God made with Abraham.
"For God hath concluded them all in unbelief, that he might have mercy upon all"
In this way, writing about the hopes and prospects for Israel, the Apostle Paul summarizes the whole divine plan of salvation, which almost from the very beginning is carried out on the basis of the Abrahamic Covenant and in connection with Israel. The goal is salvation, of both Jews and Gentiles. Although God foresaw the time of disobedience and blindness both in relation to Israel and all other nations, He did so "that he might have mercy upon all" (Rom. 11:32 KJV). The Greek eleeo occurring here in the original text is closely related to eleemosyne, which means mercy, but it can also mean charity, which clearly demonstrates not only the state in which we all are in Adam, but also the undeserved gift of justification and eternal life in Jesus Christ (Rom. 5:15-17; Rev. 3:17).
Because the disobedience Paul writes about is synonymous with sin, his final statement can also be seen in a wider context concerning the existence of evil in general. We learn in this way that it was necessary so that eternal life could be offered to man; it was a part of the process of bringing man to perfection. Although it is believed that Adam was perfect before he sinned, in fact we learn that he was 'very good' and, as it turned out, vulnerable to abandoning the commandments of God (Gen. 1:27,31). Knowing good and its consequences, as well as evil and its consequences, is necessary for man to become morally perfect, capable of choosing conduct consistent with good principles out of his own will. The Creator was not oblivious of this fact and therefore, as elsewhere Paul says, 'the creature was made subject to vanity, but on the basis of hope for the freedom to glory' (Rom. 8:19-22, see Greek text).
This freedom we will truly gain when "There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob," and subsequently from all the other people raised from the dead under the government of God's Kingdom (Rom. 11:15). It will be a time when man will be made familiar with the experience of good, just as now we are made familar with the taste of evil (Ps. 90:15). This knowledge will indeed make us 'free to glory' - anyone who will choose the good principle and will reform their characters in accordance with the requirements of God will receive the gift of eternal life. This is a great Biblical hope for all mankind, which is not taught by secular philosophy, and which was heavily distorted even within Christianity. It's a real treasure for all who have been informed of it by the Word of God (Mt. 13:45,46). Therefore, Paul concludes this section of his letter with an exclamation, which can only be quoted in full without unnecessary comments:
"O the depth of the riches both of the wisdom and knowledge of God! how unsearchable [are] his judgments, and his ways past finding out! For who hath known the mind of the Lord? or who hath been his counsellor? Or who hath first given to him, and it shall be recompensed unto him again? For of him, and through him, and to him, [are] all things: to whom [be] glory for ever. Amen." (Rom. 11:33-36 KJV).