Joar Haga traces the Lutheran doctrine of communicatio idiomatum, the exchange of properties between the natures of Christ, as it developed in the 16th and the early 17th Century. Regarding it as the nerve of his soteriology, Luther stressed the intimacy of the two natures in Christ to such a degree that it threatened to end the peaceful relationship between theology and philosophy. Earlier research identified two traditions of Lutheran Christology: One train of thought follows Luther in emphasising the difference between philosophy and theology, the other in the conservative pupils of Melanchthon.