In contrast to “those who belong to Christ” who are resurrected “at his coming,” there are those who do not belong to Christ, being those resurrected at “the end.” Just as the first two orders obviously involve people (“Christ the firstfruits” and “at his coming those who belong to Christ”), so also this third order includes people as well, though it is stated more succinctly (“the end”). Multiple points can be made to explain what Paul himself says in brief, that the resurrection includes three orders of people, the second order taking place after the first, and the third order taking place after the second.
First, as just mentioned, just as the first and second orders of the resurrection involve people, so also it is natural to conclude that the otherwise ambiguous third order called “the end” involves people as well.
Second, Paul’s use of “then…then” (epeita…eita) to introduce the second and third orders (1 Cor 15:23, 24) indicates that they each chronologically follow a similar and previously mentioned order. In fact, Paul has already used these transitional adverbs to show a chronological progression within this very chapter. In speaking to a progression of post-resurrection appearances by Christ, Paul noted that He “appeared to Cephas, then [eita] to the twelve. Then [epeita] he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. Then [epeita] he appeared to James, then [eita] to all the apostles” (1 Cor 15:5–7). The order of these terms is reversed in 1 Cor 15:5–6 (eita…epeita), and the order is the same in 1 Cor 15:7 as it is in 1 Cor 15:23–24a (epeita…eita). The terms are interchangeable and basically synonymous, which is why both can be translated “then.” If Paul meant to conflate the events of 1 Cor 15:24b–28 with “the end” of 1 Cor 15:24a, this use of adverbs makes it difficult to detect. Instead, he intentionally separates these events from one another in a chronological series, indicating that, just as there is a gap of time between the first and second orders, so also it is between the second and third.
Third, with this exegesis in hand, we can see that this interpretation of 1 Cor 15:23–24a is complemented by John’s eschatology in Revelation 20. For both authors, there is a resurrection of believers (1 Cor 15:23b with Rev 20:4–6), a kingdom ruled by Christ (1 Cor 15:24b with Rev 20:4, 6), and a subsequent resurrection of unbelievers (1 Cor 15:24a with Rev 20:5, 13). Death itself is destroyed (1 Cor 15:26 with Rev 20:14), and then the Father and Christ are enthroned over the kingdom in the eternal state (1 Cor 15:24b, 28 with Rev 21–22; cf. 22:1, 3).