Gaffin: Pentecost "once-for-all, redemptive historical"
"To that end (i.e., the worldwide, church-building spread of the gospel, anticipated throughout the entire Old Testament with its unified focus on Christ), the apostles, as witnesses, are to wait for Pentecost (vv. 48f.; Acts 1:5, 8). The Spirit's coming on Pentecost is as climactic an event, and as essential to the messianic work of salvation foreseen in the Old Testament, as are Jesus' death, resurrection, and ascension.
"Peter reinforces that point, in fact it is a major emphasis, toward the close of his (essentially Christ-centered) Pentecost sermon. In Acts 2:32-33, following out of his focus on the earthly activity, death and especially the resurrection of Jesus (vv. 22-31), he closely conjoins, in sequence: resurrection—ascension—reception of the Spirit—outpouring of the Spirit. The last, Pentecost, is coordinate with the other events, conjoined with them in an especially intimate way; it is climactic and final on the order that they are; it is no more capable of being a repeatable paradigm event then they are. Resurrection—ascension—Pentecost, though temporally distinct, constitute a unified complex of events, a once-for-all, redemptive-historical unity, such that they are inseparable; the one is given with the others.
"…Pentecost, then, is an event, an integral event, in the historia salutis, not an aspect of the ordo salutis; Pentecost has its place in the once-for-all, completed accomplishment of redemption, not in its ongoing application. Without Pentecost the definitive, unrepeatable work of Christ for our salvation is incomplete. The task set before Christ was not only to secure the remission of sin but, more ultimately, as the grand outcome of his Atonement, life as well (e.g., John 10:10; 2 Tim. 1:10)—eternal, eschatological, resurrection life, or, in other words, life in the Spirit.
"Without that life 'salvation' is obviously not only truncated but meaningless. And it is just that life, that completed salvation, and Christ as its giver that is openly revealed at Pentecost. The difference Pentecost makes is primarily a difference for Christ. Along with the resurrection and ascension, it marks him out as having received the Spirit, as the result/reward for his obedience unto death (Phil 2:8-9), in order to give the Spirit (Acts 2:33); Pentecost shows the exalted Jesus to be the messianic receiver-giver of the Spirit. -- Richard Gaffin, "Pentecost: Before and After"