Paul’s letters are the earliest Christian documents we have; he wrote during the church’s most formative years, when it still expected the Parousia to come at any moment, and we see in his letters that his thought about the afterlife progressed and matured as, contrary to his expectation, the first generation of Christians began to die. This watershed realization, that the Parousia would likely not happen before the first Christians—especially Paul himself—died, drove Paul to think more deeply about what exactly would transpire after someone dies.
Early in Paul’s career, his pareschatology is quite general: the dead in Christ are simply “asleep” until the Parousia, at which point they are “raised” or “awakened”—that is, resurrected—and God brings them with him down to the earth. In fact, Paul makes no direct mention of the intermediate state during this period; instead, we are left to infer an intermediate state through the gap in time between the believers’ deaths and the Parousia, which Paul, at this point, still expected to see in his lifetime.
Later in his career, Paul has a change of thought. He has realized that he will likely die before the eschaton, so he thinks through the nature of the intermediate state more fully. In discussing his own fate after death, he teaches that, after death, the person appears before Christ to be judged, “so that each may receive recompense for what has been done in the body, whether good or evil” (2 Cor 5:10). He expresses anxiety that he will be “found naked” and hopes instead that he will be “further clothed” with immortality (2 Cor 5:3-4). Ultimately, though, Paul is assured that he will be “with Christ” after death, because God has guaranteed it through the Holy Spirit and Christ has made Paul his own (2 Cor 5:5; Phil 1:21-23; 3:12). At this point in his career, Paul also clearly believes in an underworld where the dead await the Parousia; he cites a hymn or confession that teaches a three-story universe (heaven, earth, and underworld) and modifies a passage from Deuteronomy to discuss Jesus descending to the underworld (Phil 2:5-11; Rom 10:6-7).
Several themes are not specific to either his earlier or later periods. One such theme is that death is God’s enemy, which Jesus defeated through his resurrection (1 Cor 15:26, 54-55). Even when Paul talks about death as God’s punishment for sin, he always counters that notion with the good news that Jesus has conquered death. Throughout his career, Paul maintained that death does not separate the believer from Christ—that Christ, because he has conquered death, gives believers the power not to avoid death, but to go through death and survive it (Rom 7:24; 8:10, 13, 33-39; 1 Thess 5:9-10). Moreover, Christ, as the “the first fruits of those who have died” (1 Cor 15:20), is a signal that the rest of the dead would soon “ripen,” so to speak, and rise from the dead, as well. 
The value of the deutero-Pauline letters for this topic is that they provide clear evidence of Paul’s doctrine, as received by early Pauline Christianity. In these letters—specifically, Ephesians—the Pauline community believed that Jesus descended into the underworld after his death, which, along with Philippians 2:5-11 and Romans 10:6-7, is evidence that Paul taught the existence of an underworld where souls spend the intermediate state (Eph 4:9). This community also believed that the resurrection had already begun, in a spiritual sense, picking up on Paul’s discussion of the “spiritual body” in 1 Corinthians 15 (Eph 2:4-7). Thus, the deutero-Pauline letters serve as further proof that Paul taught an intermediate state.
1. John Hick, Death and Eternal Life (New York: Harper & Row, 1976), 187-188.
2. Interestingly, Paul refers to death as “sleep” only in 1 Thessalonians and 1 Corinthians (both of which he wrote during this period); it is highly plausible that, because of how brief he expected the intermediate state to be, he did not devote very much time—unlike his forebears and contemporaries—to thinking about what it would be like (Karel Hanhart, The Intermediate State in the New Testament (Franeker, Holland: T. Wever, 1966), 109). It is also noteworthy that, unlike Wisdom of Solomon and, later, Revelation, Paul does not refer to the soul as being asleep—for Paul, it is not the soul but the body that is asleep while the believer is in the intermediate state (N. T. Wright, The Resurrection of the Son of God (Minneapolis: Fortress, 2003), 216).
3. We may observe that Paul’s scenario of post-mortem judgment finds its root in God’s judgment of Adam and Eve in Genesis 3. Adam and Eve, originally being naked and unashamed, became aware of their nakedness after eating the forbidden fruit, so they made poor, flimsy garments for themselves from leaves. However, God, after casting judgment against them, made them superior garments from animal skin. In the same way, Paul is anxious that he, too, will be found naked, the ragged garments he is currently wearing (that is, his sinfulness and mortality) having been stripped away, but he also hopes that he will receive a superior garment (immortality) after being judged. For a later illustration of this same theme, see LAE Apoc. 22:1-23:5.
4. In Romans 10:6-7 [“But the righteousness that comes from faith says, ‘Do not say in your heart, “Who will ascend into heaven?”’ (that is, to bring Christ down) ‘or “Who will descend into the abyss?”’ (that is, to bring Christ up from the dead).”], Paul modifies Deuteronomy 30:12-13 (“It is not in heaven, that you should say, ‘Who will go up to heaven for us, and get it for us so that we may hear it and observe it?’ Neither is it beyond the sea, that you should say, ‘Who will cross to the other side of the sea for us, and get it for us so that we may hear it and observe it?’”).
5. Oscar Cullmann, Immortality of the Soul or Resurrection of the Dead? The Witness of the New Testament (London: Epworth, 1958), 24, 27.
6. We may see one such pair in Rom 5:12 and 6:23. cf. Hick, Death and Eternal Life, 207.
7. Alan F. Segal, Life After Death: A History of the Afterlife in the Religions of the West (New York: Doubleday, 2004), 437; Cullmann, Immortality of the Soul or Resurrection of the Dead, 10-11.
8. Segal, Life After Death, 403.
9. We must note that in 1 Corinthians 15, Paul provides no specific information about when the believer receives the spiritual body; this passage is thus an inference on the part of the Ephesians community, attempting to clarify what Paul left unclear.