Where did anyone say or describe this in the Bible though?
You will find
one divine Being, God, in
three separate divine persons described in the following where
both Jesus and the Holy Spirit are God, and
all three are presented in NT teaching from the beginning.
1) We have three separate persons (divine agents), Father, Son and Holy Spirit,
presented in the work of salvation:
a)--at
its beginning (Luke 1:35),
-----at the inauguration of Jesus' public ministry (Matthew 3:16-17) and
-----in the work of atonement (Hebrews 9:14),
b) the Holy Spirit completing the work (salvation) of the Father through the Son
(Acts 2:38-39; Romans 8:26; 1 Corinthians 12:4-6; Ephesians 1:3-14, Ephesians 2:13-22; 2 Thessalonians 2:13; 1 Peter 1:2),
c) the only way to enter the kingdom of the Father (salvation) is through faith in the Son and regeneration by the Holy Spirit (John 3:1-15).
2) And Jesus shows the personhoods of three distinct and separate divine agents:
The Son is
sent by the Father, in the Father's name (John 5:23, 36, 43).
The Spirit is
sent by the Father in the Son's name (John 14:26).
The Spirit is subject to the Son as well as to the Father, for the Spirit is
sent by the Son as well as the Father (John 15:26, 16:7, 14:26).
One doesn't send oneself, one sends another who is separate from oneself.
The Trinity--one God in three separate divine persons--is presented in NT teaching
from the beginning.
And which Biblical description you can be expected to deny, just as you deny the apostolic testimony to the divinity of Jesus.
It's not about lack of Biblical testimony, its about lack of belief.