In his quirky little essay on Benedict XVI, The Mystery of Evil, Giorgio Agamben cites Odo Casel's finding that “the term mysterion indicates a praxis, an action or a drama in the theatrical sense of the term as well, that is, a set of gestures, acts, and words through which a divine action or passion is efficaciously actualized in the world and time for the salvation of those who participate in it” (28).

This, he argues, is what Paul means when he speaks of mystery. For Paul, the mystery “is not a secret” but “something that is said and manifested. It is not the wisdom of God, but that by means of which this wisdom is expressed and revealed, in such a way—as happens in the mysteries—the uninitiated do not comprehend it. . . . The wisdom of God is therefore expressed in the form of a mystery, which is nothing other than the historical drama of the passion, namely, an event that really happened, which the uninitiated do not understand and the faithful grasp for their salvation” (29-30).

This is also the significance of “mystery” in what Paul describes as the “mystery of iniquity” in 2 Thessalonians 2. It's an eschatological drama, involving a katechon, a restrainer (“identified with the Empire . . . or with the the Church, in any case an  institution,” 33), and the Messiah. The man of lawlessness is “situated” between the two, and his revelation “coincides with the katechon's exiting the scene,” which precipitates a final battle. 

The Messiah and the anti-Messiah are strangely similar. The Christ overcomes the law, and thus opens up a “zone of lawlessness that coincides with messianic time and in this way frees the anomos, the outlaw” (33). While the restrainer is operative, the lawlessness of the church and of antichrist is concealed under the cover of “judicially constituted authority.” The mystery play is unveiled and begins “with the manifestation of the inoperativity of the law and with the essential illegitimacy of every power in messianic time” (34). 

In a parenthetical comment, Agamben seems to imply that this time is upon us: “this is what is happening under our noses, when the powers of state act openly as outside the law. In this sense, the anomos does not represent anything but the unveiling of the lawlessness that today defines every constituted power, within which the State and terrorism form a single system” (34). Throughout the essay, though, Agamben implies that the church is always the site of a double mystery, the mystery of the passion and the mystery of lawlessness. For example, “The ‘not yet' defines the action of the katechon, of the force that restrains; the ‘already' refers to the urgency of the decisive element” (35). It's not clear that Agamben means this literally, but perhaps he does. Perhaps he thinks that we're at an eschatological moment, when the twin mysteries opened by the passion of Christ are heading toward a final conflict. 

2 Thessalonians 2 is speaking, I think, about the first-century situation. The man of lawlessness is a rebel high priest who will take up his seat in the temple of God; the restrainer is the Roman empire that, for a time, protected the church from the worst of Jewish persecution. 

Even though Agamben misconstrues the text, he is right the church has lost its soul because she doesn't take eschatological texts seriously. Instead of seeing the mystery of iniquity as an eschatological conflict, it has been “transformed into an atemporal structure, which aims to give a theological jurisdiction to evil and, at the same time, to slow and ‘hold back' the end of days” (36). Kenotic theologians have rooted the mystery of iniquity in God Himself, “a justification of evil through kenosis,” and a position that turns “the drama of the end of days into an ontotheological structure.” Agamben is right to call this “a Gnostic gesture” or “a semi-Marcionite one” (36). According to Christianity, mysteries are events, and in the end “mystery and history correspond without remainder” (30).