Then in Tertullian, insofar as he was a Montanist, we have a fourth covenant, the covenant with the Paraclete, the Divine Spirit, which gives the new law at the end of the days. This means the history of salvation was understood as the education of mankind in terms of a law. This was a very powerful system of thought. It made it possible to understand why the Old Testament belongs to the Christian Bible, why philosophy belongs to Christianity: they all are stages in the one history of salvation; they are not negated by the revelation in Christ, but confirmed. This should never be forgotten in Christian theology, that the problem of dualism was solved in terms of a history of salvation in different covenants. One can say that it is the Biblical idea of kairos, the “right time.” At any time the revelation must do something special.