There is no question that trying to explain the doctrine of the Trinity to someone who is not a believer (or maybe is, but just doesn’t get it) is very difficult to do. How can we explain in detail the nature of God? What we can do is offer what is revealed to us through Scripture (which is all we need and definitely a lot). What is abundantly clear from examining Scripture is that the Father, the Son and the Holy Spirit are all God, yet all 3 distinct persons.
While the word Trinity is not in the Bible, it is the best, most capable theological term to explain. I would remind anyone arguing against this that the word ‘Bible’ is not in the Bible, either.
A sure sign of a cult is denial of this doctrine. If a body of believers denies the Trinity, they deny the God I know and love and are not truly Christians.
From Chapter 2, Paragraph 3 of the 1689 London Baptist Confession:
In this divine and infinite Being there are three subsistences, the Father, the Word or Son, and Holy Spirit,27 of one substance, power, and eternity, each having the whole divine essence, yet the essence undivided:28 the Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father;29 the Holy Spirit proceeding from the Father and the Son;30 all infinite, without beginning, therefore but one God, who is not to be divided in nature and being, but distinguished by several peculiar relative properties and personal relations; which doctrine of the Trinity is the foundation of all our communion with God, and comfortable dependence on Him.
Here are supporting Scriptures:
For there are three that testify: -1 John 5:7 (NIV)
Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, -Matthew 28:19 (NIV)
May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all. -2 Corinthians 13:14 (NIV)
God said to Moses, “I am who I am.This is what you are to say to the Israelites: ‘I am has sent me to you.’” -Exodus 3:14
Believe me when I say that I am in the Father and the Father is in me; or at least believe on the evidence of the works themselves. -John 14:11 (NIV)
yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist. -1 Corinthians 8:6 (ESV)
And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. 18 No one has ever seen God; the only God, who is at the Father’s side, he has made him known -John 1:14, 18 (ESV)
But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me: -John 15:26 (KJV)
And because you are sons, God has sent forth the Spirit of His Son into your hearts, crying out, “Abba, Father!” -Galatians 4:6 (NKJV)