Many Christians today suppose that all Christians have always held the Trinity to be the one and only understanding of the Godhead. This couldn't be farther from the truth. There was much debate in the early centuries concerning the nature of God and the identity of Christ.
Here are a few resources from scholarly resources and reference books. These are not written by Oneness writers.
1. The Catholic Encyclopedia: 1912 edition, volume 15, page 47 reads; “In scripture, there is yet, no single term by which the three divine persons are denoted together.”
2. The Hastings Dictionary of the Bible: 1963 edition, page 1015 reads; “The Christian doctrine of God, as existing in three persons, and one substance, is not demonstrable by logic, or by scriptural proofs, the term ‘trias’ was first used by Theophilus of Antioch in 180 A.D., although not found in scripture, it was thereafter used.”
3. The Interpreters Dictionary of the Bible: 1962 edition, volume 4, page 711 reads; “The word ‘trinity’ was first coined by Tertullian, and is not a Biblical term.”
4. The Dictionary of Theology: 1965 edition, page 447 reads; “The New Testament does not in any way speculate on the trinity (the use of the term is introduced later), but reveals the Father through the Son, in the Holy Spirit.”
5. The Encyclopedia Britannica: Volume 3, page 365-366 reads; “The trias and trinity formula was not uniformly used from the beginning, and up to the third century.”
6. The World Book Encyclopedia: Volume 16, page 7270 reads; “In 325 A.D., in the heat of debate and argument, the Trinitarian theory was first authoritatively set forth, although unexplained and unscriptural.”
7. The Encyclopedia of Religion and Ethics: 1951 edition, volume 12, page 458 reads; “Development of the Doctrine, the Old Testament could hardly be expected to furnish the doctrine of the ‘trinity’ if belief in the trinity is grounded upon the belief of the incarnation of God in Christ, and upon the experience of the spiritual redemption. In the New Testament, we do not find the doctrine of the trinity, in anything like its developed form, not even in the Pauline and Johannine theology.”
8. The Ten Epochs of Church History: Volume 3, the Ecumenical Councils page 72 reads; “To the simplest and most primitive faith, Jesus Christ was simply God, nothing less than God.”
9. The Catholic Encyclopedia: 1967 edition, volume 14, page 295 reads; “The Christian dogma of the trinity, only very slowly developed between the 3rd and 8th centuries. It was an attempt to harmonize the various passages, in which the Father, Son, and Holy Ghost are noticed in the New Testament. Only in the last quadrant of the 4th century that what might be called the definitive dogma of the One God in three persons became thoroughly assimilated into Christian life and thought.”
10. The Encyclopedia Britannica: 11th edition, volume 3, page 366 reads; “Not until the latter part of the 4th century were the Eastern churches brought into better harmony with the Roman idea of the Godhead, and the Trinitarian formula. As late as 867 A.D. however, Pope Nicholas still recognized baptism in the Name of Jesus Christ as valid. After the newly organized Roman Catholic Church was set in order with the decision made by its new head, Constantine, that the Trinity, and the Trinitarian formula only should be used. All who disagreed with this policy were branded as heretics, and many of the leaders were banished, suffering cruel punishments.”