Though he is connected to Montanism, which was, eventually, transformed into Tertullianism by 400 C.E., before the influence of Montanist-Tertullianist movement subsided altogether, Tertullian offered two very significant contributions that survived to become part of the theological and doctrinal stance of the medieval church: the Trinitarian Formula and the use of sacraments as church practice.
The Trinitarian Formula, which was first used by Tertullian during the development of the early Church, “viewed [God] as three persons in one substance.” This view of God was influenced by what is known as economic Trinitarianism, through “the belief that God the Father brought forth two hands, [which were] the Son and the Holy Spirit, to serve as mediators in creating the world.” By viewing God as three persons in one substance and explaining the meanings of “person” and “substance” in terms drawn from their legal use, this became incorporated into not only Trinitarianism but Christology, where it is evident that Tertullian “coined the formulas that would eventually become the hallmark of orthodoxy.”
What would also become just as significant a hallmark of orthodoxy is the use of sacraments, which was a term that Tertullian was the first to use and “did so to denote not only particular signs, like the water of baptism, but also the whole rite of which the sign was a part.” Since the term “sacrament,” then, “came to mean more than simply ‘sign’ or even ‘ritual’ and [to that end] carries with it the idea that sacred action changes the lives of the participants,” it came to represent the objectivity of the grace of Christ and the continuation of the basic sacramental reality of God’s manifestation in Christ. To that end, when considering religious life in the Middle Ages, sacraments were, perhaps, the most important thing in medieval church history, which are exemplified in the use of water, bread, wine, oil, a word, the laying on of hands as all becoming “sacramental if a transcendent substance is poured into them.”
 This was the name of the 2nd century C.E. movement which, along with Gnosticism, provided the early Christian church with its greatest challenge. The movement, initially led by Montanus, was otherworldly, stressed the importance of martyrdom, and awaited the coming of the Lord. Montanism, as a religious movement, forced the early Church to become Catholic. William L. Reese, Dictionary of Philosophy and Religion: Eastern and Western Thought (Atlantic Highlands, NJ: Humanities Press Inc., 1980), 367.
 What must be taken into consideration, first, is that Tertullian was part of a religious fringe movement that acted in opposition to the early Church. But, the Trinitarian speculation begins in the 2nd century, with Athenagoras (cir. 177 C.E.), who defended the dotrine as being an essential part of the Church’s faith. This stance was expounded at length by Tertullian. Sinclair Ferguson and David F. Wright, eds., New Dictionary of Theology (Downers Grove, IL: InterVarsity Press, 1988), 692.
 William L. Reese, Dictionary of Philosophy and Religion: Eastern and Western Thought (Atlantic Highlands, NJ: Humanities Press Inc., 1980), 572.
 Sinclair Ferguson and David F. Wright, eds., New Dictionary of Theology (Downers Grove, IL: InterVarsity Press, 1988), 692.
 This is, perhaps, due to the fact that Tertullian was either a lawyer or had been trained in rhetoric, and that his entire output bears the stamp of a legal mind. Justo L. Gonzalez, The Story of Christianity: The Early Church to the Dawn of the Reformation (V.1) (New York, NY: Harper Collins, 1984), 74.
 The doctrine of Christ, in terms of his person and nature. Sinclair Ferguson and David F. Wright, eds., New Dictionary of Theology (Downers Grove, IL: InterVarsity Press, 1988), 137.
 Justo L. Gonzalez, The Story of Christianity: The Early Church to the Dawn of the Reformation (V.1) (New York, NY: Harper Collins, 1984), 77.
 In its original meaning as “sacred sign,” it was used in the Roman army as the name for the oath to the Emperor. The word “sacrament” does not appear in Christian writing until the early third century. Linwood Urban, A Short History of Christian Thought (New York, NY: Oxford University Press, 1995), 255.
 Ibid., 256.
 Paul Tillich, A History of Christian Thought: From Judaic and Hellenistic Origins to Existentialism (New York, NY: Simon and Schuster, 1968), 155.
 Ibid., 154 .
 Ibid., 155.