Early Christian Apologists Bore Witness to the Apostolic Faith: 

Tertullian and Irenaeus on the Trinity and Ecclesiology

Jason Caros | January 31, 2026

The first few centuries following the establishment of the Church by The Lord and Savior Jesus Christ were characterized by rapid growth in the lands of the Roman Empire and parts of the Persian Empire, and during this time important defenders of the Christian faith made lasting contributions as they defended the Church against attacks from the outside and within. In the second century, Christian apologists[1] such as Saints Aristides, Justin Martyr and Melito of Sardis made great strides in defending the faith handed down by the Apostles and the Apostolic Fathers[2] by their lives and teachings. In the next generation, these men were followed by the likes of Saint Irenaeus, Saint Hippolytus, Tertullian, and Saint Cyprian. Each made important contributions to be sure, but for the purposes of this essay, the significance of Tertullian’s Christological contributions and St. Irenaeus’ ecclesiological contributions to the Church will be the focus.

Tertullian lived in the area of Carthage in North Africa during the latter half of the 2nd and early 3rd centuries. This region of North Africa fell under the ecclesiastical jurisdiction of Rome and was considered to be geographically and theologically Western in its approach, and it is in this milieu that Tertullian’s impact is first felt. Some have referred to him as the Father of Latin or Western Christianity and he is the first known Christian writer to produce his works in Latin as opposed to Greek, although he knew both languages. It was Tertullian who gave the Latin Church its vocabulary and fundamental priorities [Walker, p. 80], and it was him who first used the word trinitas or Trinity to describe the Godhead [Kelly, p. 151].  

Tertullian, like the apologists before him, used the power of his pen to combat heresies in the early Church such as those related to Marcion, Valentinian, and most notably the mysterious person of Praxeas, a Monarchian teacher who denied the reality of the Logos (Word)[3] as distinctive from the Father—patripassianism [Kelly, pp. 19-120]. In his influential work, Against Praxeas, Tertullian delivered a systematic doctrine of the Trinity, arguing there is one divine ‘substance’ which is administered into three distinct but continuous ‘persons,’ from the Latin, persona; these persons were Father, Son, and Spirit, an apostolic belief handed down by the Church. Furthermore, he explained that the Person of Christ is a union of two distinct, unconfused ‘substances,’ divine and human, in a single divine “Person” [Walker, p. 80]. The two substances or ‘natures’ explanation was the central feature of Tertullian’s Christology as he described the mysterious relation between the two. He is credited as being the first theologian to deal with this issue in a substantive way, no pun intended, while dealing with intriguing questions such as how The Logos became incarnate; for example, was He metamorphosed into flesh or did He clothe Himself in it? His conclusion on this question that both of Christ’s substances continued unaltered and unimpaired after the Incarnation, as summed up in the following words, anticipates later definitions by the Church: “We observe a twofold condition, not confused but conjoined, Jesus, in one Person at once God and man” [Kelly, pp.150-151].   

Another Church figure who made a lasting impact on Christianity was an Eastern Church Father named Irenaeus. Saint Irenaeus became bishop of the western city of Lyon (in modern day France) by way of Smyrna, located in Asia Minor. He made enduring contributions to Christian thought on such matters as Ancestral Sin[4] and redemption known as his ‘Recapitulation’ theology,[5] and he further influenced the Church and bolstered its ecclesiological position in light of various heresies. Living in the 3rd century, he dealt with the Gnostic heresy, as did Tertullian after him. This widespread heresy threatened to infect the Church with its unorthodox views and practices, and it took the likes of Saint Irenaeus to combat it. One of the methods the Gnostics used to perpetuate their teachings was an appeal to supposed secret knowledge and apostolic tradition they claimed for themselves. Irenaeus called them on their assertions. 

Saint Irenaeus’ answer to the Gnostics was multifaceted and can be boiled down to a few points. One was an argument based on what we know today as “Apostolic Succession.” As a former resident of Smyrna, a youthful Irenaeus enjoyed the privilege of looking up to the great bishop and martyr Saint Polycarp who was a disciple of the Apostle John. Additionally, he attested to the bishopric of Saint Clement of Rome, the 3rdsuccessor to the Apostle Peter, as having the words of the Apostle “ringing in his ears.[6] These examples helped to serve witness to his belief that there was literally an unbroken succession of bishops in the Church going back to the Apostles themselves. Unlike the Gnostics who claimed to have a secret apostolic tradition but could not present any evidence for their claim, Irenaeus’ contention could be substantiated historically as seen in the argument from his apologetic work Against Heresies: 

“Those that wish to discern the truth may observe the apostolic tradition made manifest in every church throughout the world. We can enumerate those who were appointed bishops in the churches by the Apostles, and their successors down to our own day, who never taught, and never knew, absurdities such as these men produce. For if the Apostles had known hidden mysteries which they taught the perfect in private and secret, they would rather have committed them to those to whom they entrusted the churches…” [Bettenson, pp. 74-75].

A complementary element to Irenaeus’ argument was that the Holy Spirit guided the same bishops in the line of Apostolic Succession because the message was committed to the Church, and the Church is the home of the Spirit. As Irenaeus said, “where the Church is, there is the Spirit of God; and where the Spirit of God is, there is the Church and all grace; and the Spirit is the truth.” [7]

Another point Irenaeus made in his apology against the Gnostics was that Apostolic Tradition had been deposited in written documents. That is, there was a discernible scripture, New Testament as well as Old, that Christians could claim as reliable, and the basis for acceptance of these books was apostolicity—the fact of apostolic authorship or authorship of those who followed the apostles. Although the idea that the books known today as the New Testament were inspired was not a novel one during Irenaeus’ day, he is considered to be the first Church Father to claim these writings as inspired and parallel to the Old Testament [Kelly p. 38, 56].    

Finally, Saint Irenaeus claimed that biblical exegesis was the domain of the Church, where apostolic doctrine, the key to scripture, was kept unbroken. He regarded the Church as the sole repository of truth because it had the apostolic writings, the apostolic oral traditions, and apostolic faith [Kelly, pp. 38, 192]. Through his encounters with the Gnostics, Irenaeus helped to strengthen the position of the bishops in the Church as being the guardians of the sacred trust (2 Tim. 1:14). He helped to establish a ‘rule of faith’ and an ‘ecclesiastical canon,’ so that by the 5th century the Western Father, Saint Vincent of Lerins, could securely pronounce a rule for distinguishing the truths of the Church from heresies when he said: “quod ubique, quod semper, quod ab omnibus," or “what has been believed everywhere, always, and by all.” [8]  

The impact of Tertullian’s Christology and Saint Irenaeus’ ecclesiology on the Church was large. Tertullian’s influence on the Church’s dialogue about the Holy Trinity and Person of Christ would continue into the great church councils of the 4th century and beyond, even though he eventually fell into Montanism.[9] His influence can be seen in the Nicene-Constantinopolitan Creed and writings of the Cappadocians, who as bishops could subsequently look back to Saint Irenaeus’ bolstering of the Church’s ecclesiology for their authority in determining theological matters. Saint Irenaeus’ teachings about Apostolic Succession provided a guarantee that the Christian faith of the post-apostolic era was identical with the message that the Apostles originally proclaimed [Kelly, p. 192]. It is true that a number of teachers and defenders of the faith stood out in the 2nd and early 3rd centuries but perhaps none more than the apologist, Tertullian, and the early bishop and Saint, Irenaeus, on the issues formerly discussed. 

Jason Caros is a husband, father and long-time educator who served as a classical school headmaster for twelve years. He is a cradle Orthodox Christian who has taught classes on Orthodox theology, worship, and spirituality to high school students and adults in his churches in Florida and Texas.

[1]Christian defenders were called apologists because they used the Greek term apologia (ἀπολογία), meaning a formal "speech in defense” to justify their beliefs against attacks or heresies in the early Church.

[2] Apostolic Fathers include Sts. Clement of Rome, Ignatios of Antioch, Polycarp of Smyrna, Papias of Hierapolis et al.

[3] Word is Logos (ὁ Λόγος) in John 1:1: “In the beginning was the Logos, and the Logos was with God, and Logos was God.”

[4] Known as “Original Sin” in the West. 

[5] The Greek word for Recapitulation is Anakephalaiosis (ἀνακεφαλαιώσις) which means "summing up," "reheading," or "bringing together under a head." It refers to Christ, as the new Adam, redeeming humanity by undoing the disobedience of Adam and restoring, healing, and uniting all creation in Himself. It is cosmic in nature and directly related to Theosis, the Eastern Orthodox doctrine about man’s deification and union with God. In this regard, St. Irenaeus writes, “Through his immense love God became what we are, that he might bring us to be what he is himself” (Against Heresies, V, Preface). 

[6] Against Heresies, Book III, Chapter 3, Section 3. 

[7] Ibid, Book III, Chapter 24, Paragraph 1

[8] Commonitory, Chapter II, §6. Known as The Vicentian Canon, the Commonitory was a manual written by the holy hieromonk to protect the Church against heresy. 

[9] Because of Tertullian’s later break from the communion of the Church, he is not considered a Church Father nor was he glorified as a Saint, but he is valued for his testimony to early Church practices and his writings on the Holy Trinity. 

Works Cited

Bettenson, Henry, and Maunder, Chris.  Documents of the Christian Church.  Oxford University Press, New York, 1999.

Irenaeus. "Against Heresies, Exposure and Refutation of Knowledge Falsely So-Called." Church Fathers, New Advent, www.newadvent.org/fathers/0103.htm. Accessed 31 Jan., 2026.

Kelly, J.N.D  Early Christian Doctrines.  Harper and Row, New York, 1978.

Vincent of Lerins. “Commonitory, For the Antiquity and Universality of the Catholic Faith Against the Profane Novelties of All Heresies.” Church Fathers, New Advent, https://www.newadvent.org/fathers/3506.htm. Accessed 31 Jan., 2026.

Walker, Williston.  A History of the Christian Church.  Scribner, New York, 1985.