I have become [the church’s] servant by the commission God gave me to present to you the word of God in its fullness—26 the mystery that has been kept hidden for ages and generations, but is now disclosed to the Lord’s people”—Col 1:25–26 (NIV)
Here I’ll make three observations on this text before turning to a broader reflection on biblical theology.
First, “mystery” at the beginning of v. 26 is in apposition to “the word of God” in v. 25. It is, in other words, further describing what that word is. Specifically, it’s something that had been hidden and was now revealed. Second, “the word of God” here probably refers to the gospel, which Paul has earlier described as “the word of truth” (1:5, NASB) and will later call “the word of Christ” (3:16, NASB). It’s also possible that the phrase refers to the OT, considering its use, e.g., in Rom 9:6 (“It’s not as though God’s word [cf. 9:4–5] had failed”). A decision, in fact, isn’t all that necessary in light of the way Paul describes the gospel, e.g., in 1 Cor 15:3ff.: the gospel comprises events that transpired according to the Scriptures. It’s the sort of continuity we’d expect, since both were “spoken” by the same God (see, esp., Heb 1:1–2). Third, the phrase “to present to you the word of God in its fullness” in v. 25 gives the content of Paul’s commission. Moreover, the specific task Paul had been given is more formally rendered “to fulfill [πληρῶσαι] the word of God,” even while most English versions prefer something more like what the NIV has here (see, e.g., NASB’s “so that I might fully carry out the preaching of the word of God,” emphasis original) based on the assumed equivalence of “word of God” with the gospel and the near-parallel in Rom 15:19 (“So from Jerusalem . . . to Illyricum, I have fully proclaimed [πεπληρωκέναι] the gospel of Christ”). Considering the close relationship between this “word” and the OT—whether directly (“word of God” = OT) or indirectly (“word of God” = gospel, which is in accord with the OT), it’s just possible, it seems to me, that Paul meant “fulfill” in a more overtly eschatological sense: Paul’s commission was to bring about or to completion God’s word/promise.
The implications of this text for biblical theology should be fairly clear already. Let me offer the more plausible version first. Paul says that his commission was to proclaim a message (i.e., the gospel), which was a mystery, one that had been hidden but was now revealed. Elsewhere (1 Cor 15:3ff.), Paul says this same message was in accord with the OT. Thus, Paul’s gospel was simultaneously hidden before Christ’s advent (i.e., before the “disclos[ure] to the Lord’s people”) and in accordance with the OT. Or, Paul says that his commission was to fulfill the OT, specifically a mystery, one that had been hidden—ostensibly in the OT—and only now revealed. Again, I suspect the former reading is the more likely, even while both lead to the same conclusion: what God did in Christ was anticipated in the OT, even though it was hidden until Christ’s advent. This, of course, corresponds with what Paul says, e.g., in Rom 16:25–26, where he notes that not only was his gospel previously hidden and only now revealed but also that it was revealed through the prophetic Scriptures (γραφῶν προφητικῶν). It had been, in other words, “hidden in plain view” until the Christian era. Both texts underscore, therefore, just how difficult it is summarize Paul’s hermeneutic and, more broadly, the relationship between the testaments with only one label, whether continuity (i.e., “acc. to the Scriptures”) or discontinuity (hidden/revealed), which probably explains why Paul often concludes these sorts of reflections marveling at God’s manifold wisdom (see, e.g., Rom 16:27; Rom 11:33–36; cf. also Eph 3:10).