Here is a thought arising from my preparation of material for a seminar on Preaching Luke's Gospel ...
It is widely accepted by Lukan scholars that in neither Luke's Gospel nor its sequel is there an articulation of atonement as the reason for the death of Christ on the cross. Jesus dies on the cross because the Messiah must suffer in order for God's plan for humanity to be fulfilled, but not in order that atonement may be made for sins. Naturally this leads to much pondering: how can Luke, for whom Paul is clearly a hero, be so "un-Pauline" in his theology of the cross? Quickly one can arrive at answers such as, Luke (or, maybe 'Luke' because we may have mistaken as to who the actual author of Gospel and Acts was) wrote much later than we think; he was neither a companion of Paul nor a close reader of his writings. In my own reflection built into the material I presented I proposed that Luke's primary audience, Theophilus, a godfearing Gentile, like the centurions of Luke 7 and Acts 10, had no need for certainty about forgiveness of sins, but did have a need for certainty that Gentiles are included in the plan of God; hence omission of atonement, but not for reason of Luke being ignorant or unsympathetic to Paul's theology of the cross.
Here is my thought: given the stridency in the debate between the 'Old Perspective on Paul' and the 'New Perspective on Paul', might we realistically suppose that if Luke's writings had been lost before the New Testament was formed, but then discovered in the last decade, would we welcome its discovery or discard it? My hunch is that some in the debate (i.e. some Old Perspectivers) would discard it on the grounds that it falls short of the standard set in Romans 3-8.
But the fact is: we do have Luke's Gospel and Acts in the canon of the New Testament. Does that say anything to us about the range of views on the cross which are acceptable as orthodoxy grounded in Scripture?