Luther believed that his was a restatement of the New Testament, especially of Paul. But although his message contains the truth of Paul, it is by no means the whole of what Paul said. The situation determined what he took from Paul, that is, the doctrine of justification by faith which was Paul’s defense against legalism. But Luther did not take in Paul’s doctrine of the Spirit. Of course, he did not deny it; there is even a lot of it in Luther, but that is not decisive. The decisive thing is that a doctrine of the Spirit, of being “in Christ”, of the new being, is the weak spot in Luther’s doctrine of justification by faith. In Paul the situation is different. Paul has three main centers in his thought, which make it a triangle, not a circle. The one is his eschatological consciousness, the certainty that in Christ eschatology is fulfilled and a new reality has started. The second is his doctrine of the Spirit, which means for him that the kingdom of God has appeared, that the new being in Christ is given to us here and now. The third point in Paul is his critical defense against legalism, justification by faith. (Paul Tillich, A History of Christian Thought, pp. 230-1)
Like proponents of the “new perspective” on Paul, Tillich, Lutheran theologian though he was, saw the limitations of the traditional “Lutheran” interpretation of Paul’s theology. Tillich doesn’t deny that justification by faith is present in Paul’s thought–indeed it remains very important for Tillich’s own theology. But also important is the idea that Christ inaugurates a new age and that Christians “participate,” through the Spirit, in the life of the risen Christ (or the “new being,” to use Tillich’s preferred term).
A Thinking Reed 
Never thought I’d read the New Perspective and Paul Tillich in the same breath, but I really appreciate the revisionist connection. I’d only add here that even scholarship on Luther is revising “What Luther believed” on participatory grace and Pauline Christology (as unsystematic and messy a catch-all concept that is…) beyond what might have been called the “Lutheran” perspective on Christology. A number of folks now revisit either Luther’s Christological panentheism and the indwelling of Christ in the world (ethicists like Larry Rasmussen, Cynthia Moe-Lobeda make this move). Or Luther scholarship in the vein of the Finnish School points towards a more Eastern Orthodox conception of Luther’s understanding of salvation–leaning towards deification and Christ “present in the believer through faith.” I.e., actually, ontologically present. They may overstate their case, but I think many of the points they make are right on. Regardless, most of these scholars, and the Luther scholarship more broadly, acknowledge that Spirit is far more important and decisive to Luther than earlier theologians (like Tillich) were willing to admit.
Thanks for commenting–I’m somewhat aware of the revisionist interpretation of Luther, which is part of the reason I put “Lutheran” in scare quotes. I’m more familiar with the Finnish school than Rasmussen’s and Moe-Lobeda’s work, but would be interested in following up on that. Anything in particular you’d recommend reading?