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).