To briefly summarize what was pertinent, we start with mention of the famous Greek philosopher Plato (ca. 429-347 B.C.). He believed in a divine triad of "God, the ideas, [and] the World-Spirit," though he "nowhere explained or harmonized this triad" (Charles Bigg, Christian Platonists of Alexandria, 1886, p. 249).
Later Greek thinkers refined Plato's concepts into what they referred to as three "substances"—the supreme God or "the One," from which came "mind" or "thought" and a "spirit" or "soul." In their thinking, all were different divine "substances" or aspects of the same God. Another way of expressing this was as "good," the personification of that good, and the agent by which that good is carried out. Again, these were different divine aspects of that same supreme good—distinct and yet unified as one.
Such metaphysical thinking was common among the intelligentsia of the Greek world and carried over into the thinking of the Roman world of the New Testament period and succeeding centuries. As the last of the apostles began to die off, some of this metaphysical thinking began to affect and infiltrate the early Church—primarily through those who had already begun to compromise with paganism.
As Bible scholars John McClintock and James Strong explain: "Towards the end of the 1st century, and during the 2d, many learned men came over both from Judaism and paganism to Christianity. These brought with them into the Christian schools of theology their Platonic ideas and phraseology" (Cyclopaedia of Biblical, Theological, and Ecclesiastical Literature, 1891, Vol. 10, "Trinity," p. 553).
The true Church largely resisted such infiltration and held firm to the teaching of the apostles, drawing their doctrine from the writings of the apostles and "the Holy Scriptures [the books of the Old Testament] which are able to make you wise for salvation" (2 Timothy 3:15).
Two distinct threads of Christianity split and developed separately—one true to the plain and simple teachings of the Bible and the other increasingly compromised with pagan thought and practices adopted from the Greco-Roman world.
Thus, as debate swelled over the nature of God in the fourth century leading to the Councils of Nicaea and Constantinople, it was no longer a debate between biblical truth and error. Both sides in the debate had been seriously compromised by their acceptance of unbiblical philosophical ideas.
Many of the church leaders who formulated the doctrine of the Trinity were steeped in Greek and Platonic philosophy, and this influenced their religious views and teaching. The language they used in describing and defining the Trinity is, in fact, taken directly from Platonic and Greek philosophy. The word trinity itself is neither biblical nor Christian. Rather, the Platonic term trias, from the word for three, was Latinized as trinitas—the latter giving us the English word trinity.
"The Alexandria catechetical school, which revered Clement of Alexandria and Origen, the greatest theologian of the Greek Church, as its heads, applied the allegorical method to the explanation of Scripture. Its thought was influenced by Plato: its strong point was [pagan] theological speculations. Athanasius and the three Cappadocians [the men whose Trinitarian views were adopted by the Catholic Church at the Councils of Nicaea and Constantinople] had been included among its members" (Hubert Jedin, Ecumenical Councils of the Catholic Church: an Historical Outline, 1960, p. 28).
"The doctrines of the Logos [i.e., the "Word," a designation for Christ in John 1] and the Trinity received their shape from Greek Fathers, who . . . were much influenced, directly or indirectly, by the Platonic philosophy . . . That errors and corruptions crept into the Church from this source can not be denied" (The New Schaff-Herzog Encyclopedia of Religious Knowledge, Samuel Macauley Jackson, editor, 1911, Vol. 9, p. 91).
The preface to historian Edward Gibbons' History of Christianity sums up the Greek influence on the adoption of the Trinity doctrine by stating: "If Paganism was conquered by Christianity, it is equally true that Christianity was corrupted by Paganism. The pure Deism [basic religion, in this context] of the first Christians . . . was changed, by the Church of Rome, into the incomprehensible dogma of the trinity. Many of the pagan tenets, invented by the Egyptians and idealized by Plato, were retained as being worthy of belief" (1883, p. xvi). (See "How Ancient Trinitarian Gods Influenced Adoption of the Trinity".)
The link between Plato's teachings and the Trinity as adopted by the Catholic Church centuries later is so strong that Edward Gibbon, in his masterwork The History of the Decline and Fall of the Roman Empire, referred to Plato as "the Athenian sage, who had thus marvelously anticipated one of the most surprising discoveries of the Christian revelation"—the Trinity (1890, Vol. 1, p. 574).
Thus we see that the doctrine of the Trinity owes far less to the Bible than it does to the metaphysical speculations of Plato and other pagan Greek philosophers. No wonder the apostle Paul warns us in Colossians 2:8 (New International Version) to beware of "hollow and deceptive philosophy, which depends on human tradition and the basic principles of this world rather than on Christ"!