Now I hope these interpretations have at least given you a little shock, if you run ahead without thinking about the (meaning of “person”, in the phrase “God is personal,” and “I cannot pray to a personal God”, etc… Don’t say it so easily. . . .) . . . .
It is not only true with respect to the idea of God, especially Trinity, that Tertullian gave fundamental formulas; he did it also with respect to Christology. “We see a double essence, not confused but united in one person, in God and the man Jesus.” Now in such a statement we have the formula of the doctrine of the two natures or powers of being in the one person, namely Christ. This smooth formula of Tertullian, the juristic mind, covers centuries of problems which came out after the formula was found. But his formula prevailed over against everything which followed. Here again we must be clear about the words here persona is meant as one individual face or characteristic being of personal character namely Jesus. And in this person two different powers of being are united, namely the power of being which we call Divine and that which we call human. Each of these powers is dependent; none of them is confused with the other; it has its own standing nevertheless they are united in the unity of a person. If we ask how is this possible, then we are in the later discussions to come.
The question whether the incarnation is a metamorphosis that God becomes man or the acceptance of a human essence: Tertullian decides for the second, because he is certain, as were most of the theologian s, that God is ultimately unchangeable, and that the two powers of being must be preserved. Jesus as man is not a transformed God, but he is a real man, he is true man, and therefore can be true God also. He is not a mixture. If the Logos were transfigured or transformed into something else, then He would have changed His nature, but the Logos remains Logos in the man Jesus. So he decides much more in the line of adopting of a human nature by the Logos, instead of a mythological transmutation idea.
The saving power, according to Irenaeus, is the Divine Spirit who dwells in the Church and renews the members out of what is old, into the newness in Christ. He gives them life (zoe) and light (phos) He gives them the new reality. This is God’s work in man, which is accepted by faith. Therefore no law is needed, since we love God and the neighbor. This is the Pauline element, but it is not strong enough to overcome the anti-Pauline elements. Finally, the New Being is mystical-ethical. It is in this sense the highest form of early Catholicism, but it is not Protestantism, where the renewal is by justification through faith.
Irenaeus thinks of the process of salvation in terms of a mystical regeneration into immortality. Against this, Tertullian speaks of a wholesome discipline as the content of the Christian life. He speaks of a process of education by the law, and the reality of obedience to it is eternal life. Here we have the Roman who is a jurist and likes the law, and at the same time the ascetic pietist, who became a Montanist. We have in Irenaeus mystical participation; and in Tertullian subjection to the law: the two sides of early Catholicism, the two sides which were always effective. The second was decisive, before the Protestant break. But the Protestant break denied also the Irenaean form and returned to the one side of Paul, namely justification by faith. So we have always similar problems arising as early as that. We have the relationship to Christ more spiritual mystical participation, more legal by accepting Him as the new law. And these two sides are going on also in Protestantism.