A Brief Biblical Defense of the Holy Trinity 

(1) There is ONLY ONE GOD

"the Lord our God is ONE Lord" (Deut 6:4; 4:35,39; 32:39; Mk 12:29)

"I am the Lord and there is no other" (Isa 45:5; 43:10; 44:6,8; etc)

"there is no God besides Me" (2 Sam 7:22; 22:32; Neh 9:6; Jer 10:10-11)

"there is ONE God" (Rom 3:30; 1 Cor 8:6; 1 Tim 2:5; James 2:19)

"there is no other God but ONE" (1 Cor 8:4; Gal 3:20; Eph 4:6)

"the ONLY true [wise] God" (John 17:3; Rom 16:27; 1 Tim 1:17; Jude 25)

(2) There are "so-called" gods that are not really "gods" at all

(Jer 10:10-11; Acts 14:11-15; 1 Cor 8:4-6; 2 Cor 4:4; Gal 4:8; etc)

(3) The Father is THE God

(Matt 6:9; 11:25; John 17:3; 2 Pet 1:17; etc)

(4) The Son is THE God

(John 1:1ff; 5:18; 8:58; 10:30-33; 20:28; Matt 1:23; Rom 9:5; Col 1:16-17; 2:9; Titus 2:13; Heb 1:8; etc)

(5) The Holy Spirit is THE God

(Acts 5:3-4 "lie to the Holy Spirit....lied to GOD"; 2 Cor 3:17 "the Lord is the Spirit")

(6) The Father, the Son, and the Holy Spirit are personal and DISTINCT

(Matt 3:16-17; 28:19; 2 Cor 13:14; John 14:16; 17:5; etc)

(7) Therefore, the ONE GOD is THREE PERSONS, the Father, the Son, and the Holy Spirit according to the Bible

For more advanced discussion of the Biblical texts, doctrinal development, Trinitarian terminology, and history of the early Catholic Christian Creeds and Councils, please consult the following books:

Fundamentals of Catholic Dogma by Ludwig Ott (Tan Books, 1974) has an excellent section on the Trinity, covers the Bible and the anti-Trinitarian heresies in the history of the Catholic Church

Theology for Beginners and Theology and Sanity by Frank Sheed, a classic Catholic apologetic that presents one of the best explanations of the Trinity

Why You Should Believe in the Trinity by Robert Bowman (Baker Books, 1989), an Evangelical author responds to a Jehovah's Witnesses booklet, covers the Bible and the Fathers

God in Three Persons by E. Calvin Beisner (Tyndale House, 1984), an Evangelical author covers the Bible, the Creeds, and the development of the doctrine

The Forgotten Trinity by James R. White (Bethany House, 1999), the Reformed Baptist apologist covers the major Biblical texts in response to modern anti-Trinitarians