Seitz sees the relationship between the two Testaments as dialectical, not one way. Both testaments make a unique witness to God in their own idiom, and yet both need to be read together, the Old in the New and the New in the Old. The book of Isaiah, for example, is read in the NT in a certain way, but this reading is not exhaustive. The NT reading needs to be balanced by the OT, in order for the fullness of the reality of God to come to light. As a case study, Seitz takes the pairing in the Lectionary of Isaiah’s Song of the Vineyard (5:1-7) with Matthew’s parable of the Vineyard (21:33-43).
A reading of the NT passage on its own, apart from the OT, could lead to the belief that that vineyard has been taken from Israel and given to the church (v. 43). The quote in v. 44 of Isa 8:14 would reinforce the view that it is the Jewish opponents that are crushed. Even when read in conjunction with the OT passage, a hermeneutic which privileges the NT would lead to the conclusion that just as Israel failed in Isaiah’s day, so it failed in Jesus’ day.
However, a reading which refuses to privilege either Testament, but instead tries to hear them both dialectically as Scripture, is forced to ask the question of whether the vineyard’s new tenants, the church, will produce fruit and be worthy to be stewards of Christ. To quote:
Isaiah’s song of the vineyard is now not just addressed to Israel as past historical referent, but as a word of Christian scripture, heard in the light of the New, it becomes a word of prophetic address to another “Israel” (Gal.6:6), the new tenants, the church (Word Without End, 225).Matthew’s concluding reference to Isa 8:14 becomes a warning to anyone, ruling out a supercessionist reading.
This small example illustrates what Seitz means when he says “hearing the Old in light of the New means also hearing the New in light of the Old”. This dialectical approach understands the relationship between Old and New to be reciprocal rather than developmental.