The Latin author Tertullian (155 AD – 240 AD) introduced the term "Persona" into theology. He is the first writer to use the word "Trinitas" in Latin. He claims that the persons of the Holy Trinity can be counted in such a way that their "distinctio" (distinction) and "dispositio" (classification) do not create a "separatio" (division) among them. The three divine persons are One "unus," yet they do not constitute a single individual "unum."

In the history of doctrines, Tertullian has the great merit of having provided theology with the foundations for the correct formulation of the Trinitarian doctrine: "una substantia – tres personae," emphasizing both the consubstantiality of the three divine persons and their substantial distinction, which indeed had some influence on the formulation of the Nicene Creed in 325 AD.

Although he went through four different phases of his life, from the "orthodox phase" to the full "Montanist phase," there seems to be no negative impact on his theology, as evidenced by his doctrinal work Against Praxeas (Adversus Praxean), a gem of Trinitarian theology.