Many Church historians agree that the doctrine of the trinity did not exist as we know it today in the immediate post-apostolic age. The Christian leaders following the apostles did not allude to a trinity, but rather they affirmed their belief in the monotheism of the Old Testament and accepted without question the deity and the humanity of Jesus Christ. Since these leaders emphasized the doctrines associated with Oneness, it can be assumed that the post-apostolic church accepted the oneness of God.
The most prominent post-apostolic fathers were Hermas, Clement of Rome, Polycarp, and Ignatius. Their ministries spanned the time from about 90 to 140 A.D.
Irenaeus, a prominent Christian leader who died around 200 A.D., had an intensely Christocentric theology and a firm belief that Jesus was God manifested in flesh. He held that the Logos which became incarnate in Jesus Christ was the mind of God, and was the Father Himself.
Some scholars classify Irenaeus as a believer in the “economic trinity.” This view holds that there is no eternal trinity but only a temporary one. It is very probable, therefore, that Irenaeus believed in a trinity of God’s activities or roles rather than a trinity of eternal persons, and he expressed some Oneness concepts. He certainly did not articulate the later trinitarian dogma of three distinct co-equal persons.
We find no references to the trinity as such in the early post-apostolic writings; they refer only to one God and to Jesus as God. Modalistic monarchianism is the term most often used by church historians to refer to the Oneness view. Encyclopedia Britannica defines it as follows: “Modalistic monarchianism, conceiving that the whole fulness of the Godhead dwelt in Christ, took exception to the ‘subordination’ of some church writers, and maintained that the names Father and Son were only different designations of the same subject, the one God, who ‘with reference to the relations in which He had previously stood to the world is called the Father, but in reference to His appearance in humanity is called the Son.'” The most prominent modalist leaders were Noetus of Smyrna, Praxeas, and Sabellius. Noetus was Praxeas’ teacher in Asia Minor, Praxeas preached in Rome about 190, and Sabellius preached in Rome about 215. Since Sabellius was the best known modalist, historians often call the doctrine Sabellianism. Sabellius relied heavily upon Scripture, especially passages such as Exodus 20:3, Deuteronomy 6:4, Isaiah 44:6, and John 10:38. He said that God revealed Himself as Father in creation, Son in incarnation, and Holy Ghost in regeneration and sanctification. Some interpret this to mean that he believed these three manifestations were strictly successive in time. If so, he does not reflect the beliefs of older modalism or of modern Oneness.
Encyclopedia Britannica describes Sabellius’ belief in this way: “His central proposition was to the effect that Father, Son and Holy Spirit are the same person, three names thus being attached to one and the same being. What weighed most with Sabellius was the monotheistic interest.”
We get much of our information on the modalists from Tertullian (died c. 225), who wrote a treatise against Praxeas. In it he indicated that during his ministry “the majority of believers” adhered to the Oneness doctrine.
“The simple, indeed (I will not call them unwise and unlearned), who always constitute the majority of believers, are startled at the dispensation (of the Three in One), on the very ground that their very Rule of Faith withdraws them from the world’s plurality of gods to the one only true God; not understanding that, although He is the one only God, He must yet be believed in with His own economy. The numerical order and distribution of the Trinity, they assume to be a division of the Unity.”  We have found evidence of many other Oneness believers throughout church history in addition to those described in the research paper presented in this chapter. We feel that the believers we have discovered represent only the tip of the iceberg. Some writers find evidence that the Oneness doctrine existed among the Priscillianists (c. 350 – c. 700), Euchites (c. 350 – c. 900), and Bogomils (c. 900 – c. 1400). It appears that most Oneness believers did not leave a written record. Others had their written works destroyed by victorious opponents. Many were persecuted and martyred, and their movements were destroyed by official Christendom. We do not know how many Oneness believers and movements were not recorded in history, or how many so-called heretics were really Oneness believers. What we find, however, reveals that the Oneness belief survived in spite of its violent opposition.
In the Middle Ages, the prominent scholar and theologian Abelard (1079 – 1142) was accused of teaching Sabellian (Oneness) doctrine. Eventually his enemies forced him to retire from teaching. He sought refuge at a monastery in Cluny, France, and there died.
The Reformation produced many who opposed the doctrine of the trinity in favor of the Oneness belief. One prominent antitrinitarian at the time of the Reformation was Michael Servetus (1511 – 1553), an eminent physician from Spain. He had only a few followers, although some historians consider him to be a motivating force for the development of Unitarianism. However, he definitely was not Unitarian, for he acknowledged Jesus as God. The following description of him clearly indicates he was a true Oneness believer: “The denial by Servetus of the tripersonality of the Godhead and the eternality of the Son, along with his anabaptism, made his system abhorrent to Catholics and Protestants alike, in spite of his intense Biblicism, his passionate devotion to the person of Christ, and his Christocentric scheme of the universe.”
Servetus wrote, “There is no other person of God but Christ… the entire Godhead of the Father is in him.” Servetus went so far as to call the doctrine of the trinity a three-headed monster. He believed it necessarily led to polytheism and was a delusion from the devil. He also believed that because the church accepted trinitarianism, God allowed it to come under the rule of the papacy and so to lose Christ. He could not understand why the Protestants would come out of Catholicism but still insist upon retaining the nonbiblical and man-made doctrine of the trinity.
Servetus was burned at the stake in 1553 for his Oneness belief, with the approval of John Calvin (although Calvin would have rather had him beheaded).
Emmanuel Swedenborg (1688 – 1772) was a Swedish philosopher and religious writer who expressed a good understanding of the oneness of God. He taught a number of other doctrines that are very different from what we believe, but he did have a revelation of who Jesus really is. He used the term trinity but said it was only “three modes of manifestation” and not a trinity of eternal persons. He used Colossians 2:9 to prove that all the “trinity” was in Jesus Christ, and he referred to Isaiah 9:6 and John 10:30 to prove that Jesus was the Father. He denied that the Son was begotten from eternity, holding the view that the Son of God was the humanity by which God sent Himself into the world. He also believed that Jesus was Jehovah God who assumed humanity in order to save mankind. Swedenborg wrote: “Whoever does not approach the true God of heaven and earth, cannot have entrance into heaven, because heaven is heaven from that one only God, and that God is Jesus Christ, who is Jehovah the Lord, from eternity the Creator, in time the Redeemer, and to eternity the Regenerator: of consequence, who is at once Father, Son, and Holy Spirit and this is the Gospel which is to be preached.” He saw God (Jesus) as composed of Father, Son, and Spirit just as man is composed of soul, body, and spirit – an analogy not particularly appropriate. However, Swedenborg’s explanation of the Godhead is strikingly similar to that of modern Oneness believers.
The nineteenth century saw the emergence of Oneness writers. A Oneness believer in America was a Presbyterian minister named John Miller. In his book, Is God a Trinity? written in 1876, he used terminology slightly different from that of modern Oneness writers, but the beliefs he expressed are basically identical to those of Oneness believers today. It is amazing to read his book and see how closely he parallels modern Oneness teaching, including his treatment of Matthew 28:19. Miller believed that the doctrine of the trinity was not biblical and that it greatly hindered the church in reaching out to Jews and Muslims. He emphatically declared the full deity of Jesus Christ.
Oneness believers also existed in nineteenth century England. David Campbell reported finding a book written in 1828 that taught Oneness. The author was John Clowes, pastor of St. John’s Church in Manchester.
In the twentieth century, the most significant Oneness force has been the Oneness Pentecostals, although some scholars classify the noted neo-orthodox theologian Karl Barth as modalist (Oneness). Charles Parham, the first leader in the twentieth century Oneness movement, began to administer water baptism in Jesus’ name, although he apparently did not link this practice with an explicit denial of trinitarianism. After 1913, many rejected the trinitarianism and the trinitarian baptismal formula, thus beginning the modern Oneness movement.
A number of Oneness organizations exist today that are not Pentecostal in name, but still remain true to the Acts 2:38 message. “Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.” Many preach the one true God and His being name one as foretold in Zechariah 14:9. “And the LORD shall be king over all the earth: in that day shall there be one LORD, and his name one. ‘ There’s but one Lord and God; and Jesus is His name (Deuteronomy 6:4; Acts 10:36; Colossians 3:17). Salvation consists applying the grace of God by faith unto your baptism in His name. One must keep the whole word of God, and repent of all sin committed. He’s faithful and Just to forgive us then. The speaking of tongues is a spiritual gift given by God, but not required by Him for salvation.