The following information taken from Unger's Bible Dictionary is helpful in answering that puzzling question:
"The word Trinity ( Trinitas ) was first employed by Tertullian (2nd century), though his word was only the Lat. translation of the Gk. trias, employed by Theophilus of Antioch. The word Person was also first employed by Tertullian, though he used it in the inadmissible sense of individual.
"The Council of Nicea (A.D. 325) was an epoch in Christian history. The heresy of Sabellius and Paul of Samosata, that refused to recognize the Father as in any personal sense distinct from the Son and the Holy Spirit, had been previously condemned. But Arius, who began with the Sabellian idea that the Trinity is only one of manifestation, changed his position and declared that there were three Persons in God, but that these three were unequal in glory . . . The Council of Nicea, in opposition to Arianism and various other theories, adopted the formal statement of the consubstantiality [of one and the same nature] of the Father, the Son, and the Holy Spirit, while maintaining the distinction of personality. The doctrine of the Nicene Council was reaffirmed in various succeeding councils and is the generally recognized doctrine of the Christian church" (1988, "Trinity").
As you can see, the idea of three persons in the Godhead evolved over the first few centuries of Christianity, somewhat in reaction to popular ideas perceived as threats to the truth. Well-meaning attempts to "protect the truth" actually contradicted the Scriptures. The doctrine of the Trinity is not, as our literature explains, a teaching of the Bible.
For More Information, see our booklet Who Is God?