His main contention is that even when God hadn't clearly revealed to Israel that there was an afterlife, he had given the Israelites categories to make sliding into this doctrinal belief very easy. Wright traces the development of Israel's idea from belief that:
1) When you die you're dead spiritually, physically, the whole nine yards.
TO...
2) When you die, you're still alive spiritually and can be spiritually blessed, or spiritually cursed.
TO...
3) When you die, if you are one of God's people, you will be raised from the dead in a physical body, i.e., resurrection.
Wright says,
It would be easy, and wrong, to see the hope for resurrection as a new and extraneous element, something which has come into ancient Israelite thinking by a backdoor or roundabout route. (121)
In other words, this is not a move away from the hope which characterized all of ancient Israel, but a reaffirmation of it. It is a reaffirmation, indeed, in a way which the hope simply for a blessed but non-bodily personal life after death (as perhaps witnessed by Psalm 73 and one or two other passages) would not be. (122)
We might suggest that the likely turning point in the sequence--the moment when somebody really begins to think in terms of human beings themselves actually dying and actually being given a newly embodied life at some point thereafter--is to be found in Isaiah's servant passages. (123)
The idea that God reveals Himself and His eternal plan progressively throughout redemptive history in the canon as opposed to all at once is the foundational idea for the discipline of biblical theology. It's interesting that B.B. Warfield basically argued the same way for recognition of God's triunity as Wright does here for belief in resurrection. Warfield said that although we don't have reason to think Israel believed in a Three-Person God, the Lord had at least prepared them with the main categories they would need--namely, that the way God revealed Himself to them was multi-faceted and yet, 'the Lord your God is one'!