I think it is fair to say that most of those who profess to be Christians believe in a triune Godhead. It is a belief that is taught in the vast majority of churches today; yet it has not always been so. Jesus did not teach it, nor did those he taught. It is a belief that crept into the churches slowly, beginning in the second century and finally caught on in the fourth century.

The Catholic Encyclopedia online in an article entitled “The Blessed Trinity” we find:

In Scripture there is as yet no single term by which the Three Divine Persons are denoted together. The word trias (of which the Latin trinitas is a translation) is first found in Theophilus of Antioch about A.D. 180. He speaks of “the Trinity of God [the Father], His Word and His Wisdom (“Ad. Autol.”, II, 15). The term may, of course, have been in use before his time. Afterwards it appears in its Latin form of trinitas in Tertullian (“De pud.” c. xxi). In the next century the word is in general use. . . .It is manifest that a dogma so mysterious presupposes a Divine revelation . When the fact of revelation, understood in its full sense as the speech of God to man, is no longer admitted, the rejection of the doctrine follows as a necessary consequence. For this reason it has no place in the Liberal Protestantism of today. The writers of this school contend that the doctrine of the Trinity, as professed by the Church, is not contained in the New Testament, but that it was first formulated in the second century and received final approbation in the fourth, as the result of the Arian and Macedonian controversies. In view of this assertion it is necessary to consider in some detail the evidence afforded by Holy Scripture . Attempts have been made recently to apply the more extreme theories of comparative religion to the doctrine of the Trinity, and to account for it by an imaginary law of nature compelling men to group the objects of their worship in threes. It seems needless to give more than a reference to these extravagant views, which serious thinkers of every school reject as destitute of foundation.

The article then goes on to a section entitled “Proof of Doctrine from Scripture” followed by a section entitled “Proof of Doctrine from tradition”. Without mentioning all the scriptures in the article, I can think of only two in all of the Bible that specifically mention a triune formula. The first of course is the baptismal commission by Jesus in Matthew 28.

Matthew 28:19 (KJV)
Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost:

The Theological community and biblical scholars are divided as to the credibility of this verse. Many believe it was a later addition while others accept it as the actual words spoken by Jesus. While my personal belief sides with the former, I have been unable to amass enough evidence to prove it conclusively. This I will say, if Jesus ordered baptisms to be performed in the name of the Father and Son and of the Holy spirit, his apostles disobeyed him and baptized in the name of Jesus only, as we see from every written example in the new testament.

The second scripture which mentions a triune formula is in 1 John.

1 John 5:7-8 (KJV)
For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one.

This example is an outright fraud and scholars have known it for centuries, yet we hear it cited in sermons even today. Either pastors using this passage are ignorant of the truth or downright deceptive. The words “in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth” were not in the original text and did not show up in Bibles until the sixteenth century. They were added by some dishonest scribe, no doubt to support the trinity belief. They have not been included in modern translations.

When it comes to tradition, Jesus was emphatic that substituting it for God’s word reduced the effectiveness of his word and is a rejection of his commandments.

Mark 7:9 (KJV)
And he said unto them, Full well ye reject the commandment of God, that ye may keep your own tradition.

Mark 7:13 (KJV)
Making the word of God of none effect through your tradition, which ye have delivered: and many such like things do ye. 

For a comprehensive and insightful article on this subject you may want to read “Did the True Church Ever Teach a Trinity?” written by Dr Bob Thiel.