In spite of these assumptions, the word Trinity doesn't appear anywhere in the Bible. In fact, it did not come into common use as a religious term until several centuries after the last books of the Bible were completed.
Notice this admission in the New Bible Dictionary: "
The term 'Trinity' is not itself found in the Bible. It was first used by Tertullian at the close of the 2nd century, but received wide currency [common use in discussion] and formal elucidation [clarification]
only in the 4th and 5th centuries" (1996, "Trinity," emphasis added).
The dictionary goes on to explain that "
the formal doctrine of the Trinity was the result of several inadequate attempts to explain who and what the Christian God really is . . . To deal with these problems the Church Fathers met in 325 at the Council of Nicaea to set out an orthodox biblical definition concerning the divine identity." However, it wasn't until 381, "at the Council of Constantinople, [that] the divinity of the Spirit was affirmed" (ibid.).
We see, then, that the doctrine of the Trinity wasn't formalized until long after the Bible was completed and the apostles long dead in their graves. It took later theologians several centuries to sort out what they believed concerning the Holy Spirit.
And by no means are theologians' explanations of the Trinity doctrine clear. A.W. Tozer, in his book The Knowledge of the Holy, writes that the Trinity is an "
incomprehensible mystery" and that attempts to understand it "
must remain forever futile." He admits that churches, "
without pretending to understand," have nevertheless continued to teach this doctrine (1961, pp. 17-18).
Unger's Bible Dictionary, in its article on the Trinity, concedes that the Trinitarian concept is humanly incomprehensible: "
It is admitted by all who thoughtfully deal with this subject that the Scripture revelation here leads us into the presence of a deep mystery; and that all human attempts at expression are of necessity imperfect" (1966, p. 1,118).
Why do even those who believe in the concept of the Holy Spirit as a third person of a supposedly triune Godhead, along with God the Father and Jesus the Son, find it so difficult to explain?
Because the Bible does not teach it! One cannot prove something from the Bible that is not biblical. The Bible is our only reliable source of divine revelation and truth, and the Trinity concept simply is not part of God's revelation to humankind.
The Holy Spirit, rather than a distinct person, is described in the Bible as being God's divine power (see "Is the Holy Spirit a Person?"). To learn much more about this, read the study aid Is God a Trinity?