We believe in the Blessed Trinity because we believe in Jesus, Who revealed the Trinity Godhead.
Sorry this is a misnomer for the word Trinity is not in the Bible!
Col. 2
4 And this I say, lest any man should beguile you with enticing words.
5 For though I be absent in the flesh, yet am I with you in the spirit, joying and beholding your order, and the stedfastness of your faith in Christ.
6 As ye have therefore received Christ Jesus the Lord, so walk ye in him:
7 Rooted and built up in him, and stablished in the faith, as ye have been taught, abounding therein with thanksgiving.
8 Beware lest any man spoil you through philosophy and vain deceit, after the tradition of men, after the rudiments of the world, and not after Christ.
9 For in him dwelleth all the fulness of the Godhead bodily.
10 And ye are complete in him, which is the head of all principality and power:
Jesus revealed that God was a Trinity of Persons, but in an indirect manner, implicitly found within the Scriptures as interpreted by the Church. We can point to passages that "show" the Trinity, but they rely on a certain paradigm, a mind set that sees a verse to be read a certain way. Thus, the Bible alone is not of much help in coming to the Doctrine of the Trinity.
Regards