“substance.” It is possible for several persons to share one substance, or for one person to have more than one substance—and this is the core of Tertullian’s doctrine regarding not only the Trinity, but also the person of Christ. On the basis of this concept of substance and person, Tertullian affirms the unity of the Father, the Son, and the Holy Spirit without denying their distinction: the three share in a single and undivided substance, but this does not prevent them from being three different persons.