That which was once hidden (“hidden since the foundation of the world”) but which has been revealed is not an esoteric secret on the order of Scientology (how to “go clear” of the body) or Mormonism (induction into the secrets of the Mormon Temple), or a secret on the order of the Gnostic mystery cults (a secret knowledge or experience), but it is a secret like Poe’s purloined letter – hidden in plain sight. It is a secret hidden in plain sight in the Old Testament, in the parables of Jesus, and in human experience. This mystery is one we inhabit in the way we organize ourselves into nations and religions, it is a mystery of interpretation (of the Old Testament but of reality in general), it is a mystery concerning the relationship between creation and Creator. Paul depicts the opening of this secret or the passage from “once hidden” to “now revealed” as marking a new historical consciousness as to the purposes of creation.
According to Ephesians, it pertains to “things in heaven” and “things on earth” and to God’s predetermined purposes for all things. Paul will refer to the broad sweep of history in Romans 9-11 as the unfolding of this mystery and he will refer to the breaking down of the “dividing wall” between Jews and Gentiles as pertaining to a fulfilled cosmic order previously hidden (Eph 2:14). This reference to the breaking of a literal wall in the Temple taken as a cosmic representation, such that divided people will be made one but also that the divide between heaven and earth will be broken down, is itself a deployment of the revealed hermeneutic apart from which the mystery remained. Paul’s allegorizing or spiritualizing interpretation of the most sacred precincts (the literal inner core) of Judaism (a mode he will apply to Hebrew Scriptures) pertains cosmically and personally. People are reconstituted as a singular family in which their personhood involves a new consciousness – holistic and personal. This new family fulfills the temple purposes of the cosmos in which heaven comes to earth: “in whom the whole building, being fitted together, is growing into a holy temple in the Lord, in whom you also are being built together into a dwelling of God in the Spirit” (Eph 2:21–22). Gentiles and Jews are no longer divided, the individual is no longer divided and heaven and earth are no longer divided in this fulfilled cosmic arrangement. As Paul describes it in Galatians, the binaries (Jew/Gentile, slave/free, male/female), which are not simply a convention of language but a mode of identity and understanding, no longer pertain in this new mode of identity and thought.
The scheme of “once hidden” and “now revealed,” in taking in the full scope of history, may encompass “the age of the cosmos” in which people were “dead in their trespasses” (Eph 2:1-2), but is the mystery of that former age constituted by “the prince of the power of the air, of the spirit that is now working in the sons of disobedience” (2:2). Is the mystery simply a result of sin’s deceit or darkness? If the mystery is equated with the darkness of sin, then the mystery revealed would be reduced to the overcoming of sin (the reduction of some theological systems). But Paul connects the mystery to two epochs of history inclusive of creation and its fulfillment, such that the mystery or the concealment disclosed by the revelation of the Gospel is part of the divine plan.
In Ephesians 5 Paul connects the mystery to a primal goodness which precedes sin; which is not to say that Paul equates the mystery with the one-flesh relationship of marriage (described in Gen 2:24 and which he quotes) but the unity or oneness of the marriage relationship partakes of the mystery unfolded or fulfilled in Christ (5:31-32). Like the valence between creation and fulfillment, the once hidden significance of marital oneness is disclosed in the relation between Christ and the Church – an order inclusive of all humanity. It is not that the union between Christ and the Church, like the unity of marriage, is incomprehensible. What is revealed in this union, is the cosmic breadth of the marriage like unity brought about in Christ. Creations purpose remained an undisclosed and unfulfilled mystery which is now disclosed (made known, preached, realized, in a new unity) and realized in the Spirit.
This “Spiritual” understanding penetrates or unveils the mystery at two key junctures: the mystery of the Anointed “has now been revealed to His holy apostles and prophets in the Spirit” (Eph 3:5); and this revealing works on the inward person “who is strengthened with power through His Spirit in the inner man” (3:16) so as to plumb new depths of comprehension (3:18-19). Spirit is interwoven with the realization of the revealed mystery in each of its appearances in Ephesians (1:11; 3:16; 5:26 speaks of a spiritual washing; and 6:19 evokes the power of the Spirit to work in Paul in making the mystery known) so that it is clear that the Spirit is the means to unity – inward and outward, cosmic and local. The unity of Christ and the Church, the unity of Jews and Gentiles, the unity realized in “the inward man,” is a reconciliation in Christ sealed by the Spirit summed up as peace. This peace is not simply an interlude between wars but is a state of unity and participation in God. Being “in Christ” means participation in the cosmic plan unfolding in a unified humanity founded in peace.
All of this seems to refer back to the fact that, “He made known to us the mystery of His will” (Eph 1:9). The Gospel is nothing less than an opening up of the will of God to human understanding. We now understand what God has foreordained or predestined for the world through his Son. Strange then that there is another gospel which claims that God’s plan or his reasons for predestination are wholly internal to his being and are opaque to humanity – completely incomprehensible.
Calvin maintains that God’s predestination is mysterious and “utterly incomprehensible.” He believed this impenetrable mystery will inspire wonder and reverence in that confounding people, God’s mysterious decrees will be revered in their “wonderful depth.” Calvin warns in the opening of his chapter on predestination that we must restrain curiosity and not ask after the secret things of God, as these are forbidden. “Let us not be ashamed to be ignorant in a matter in which ignorance is learning. Rather let us willingly abstain from the search after knowledge, to which it is both foolish as well as perilous, and even fatal to aspire.” In this alternative gospel, Calvin determined (from Ephesians 1:4) that there is no passage from hidden mystery to mystery disclosed; rather God’s mysterious predestination is to elect some (and to damn others) and this election is equated with holiness. There is no room for living out this inward and outward unity, lest these achievements be confused with meritorious works. For Calvin then, the gospel is not so much a mystery revealed as a mystery compounded. The question is if a gospel that misses Christ’s disclosure and fulfillment of cosmic purposes, preordained before the foundation of the world, qualifies as Gospel, or is it in fact a counter-gospel or anti-gospel?
In Paul’s depiction the Gospel is a revealing of God’s purposes for all of creation. In Romans, Paul equates “the Gospel of God” with that “which he promised beforehand through his prophets in the holy Scriptures” (Ro 1:1-2) – the Hebrew scriptures. In the scriptures the Gospel was present but concealed until Christ retroactively brings out or delineates their prophetic element. As T.J. Lang puts it, “This does not diminish the revelatory function of the scriptures; it simply means that the Christ event is hermeneutically determinative, restructuring the perception of reality on either side of its occurrence.” In Paul’s depiction, what Christ does for the Hebrew scriptures, is what he does for all of creation and for the Temple (a microcosm) and its religion. Just as the secret of the Old Testament is disclosed in Christ, so to Christ becomes the hermeneutic key for understanding human and cosmic purposes. It is not only the scriptures but God’s will for time, for all reality that are summed up or opened up in Christ: “He made known to us the mystery of His will, according to His kind intention which He purposed in Him with a view to an administration suitable to the fullness of the times, that is, the summing up of all things in Christ, things in the heavens and things on the earth” (Eph 1:9-10). The mystery disclosed pertains to all things and this is the Gospel.
 Calvin, Institutes Book 3 chapter 21.
 T. J. Lang, Mystery and the Making of a Christian Historical Consciousness: From Paul to the Second Century, https://core.ac.uk/download/pdf/37749844.pdf