The concept of God as a Trinity can be proved from the scriptures and does not need to be taken in faith. Scripture is both Old Testament and New Testament of course.
From 2 Timothy 3:16-17 “All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, that the man of God may be complete, equipped for every good work.”
From 1 Thessalonians 5:21 “but test everything; hold fast what is good.”
First, recall what Christ tells the Samaritan woman in John 4:22-23 “You worship what you do not know; we worship what we know, for salvation is from the Jews. But the hour is coming, and is now here, when the true worshipers will worship the Father in spirit and truth, for the Father is seeking such people to worship him.” Also, from Acts 4:12 “And there is salvation in no one else, for there is no other name under heaven given among men by which we must be saved.” Hence, we are saved only through Jesus Christ who prior to his time on Earth as mortal and Divine was the Word. From John 1:1-3 “In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through him, and without him was not anything made that was made. Jesus Christ at his time on Earth was a Jew and yet Divine of course. Then resurrected and returned back to the Father in third heaven.
If you need further proof that Jesus Christ was Divine, let’s refer to Old Testament:
Zechariah 12:1,10 “The oracle of the word of the Lord concerning Israel: Thus declares the Lord, who stretched out the heavens and founded the earth and formed the spirit of man within him;” ““And I will pour out on the house of David and the inhabitants of Jerusalem a spirit of grace and pleas for mercy, so that, when they look on me, on him whom they have pierced, they shall mourn for him, as one mourns for an only child, and weep bitterly over him, as one weeps over a firstborn.” So, Jesus Christ is Lord for sure for Zechariah’s prophecy was fulfilled when Christ was crucified and then resurrected.
Is this Lord also God? Of course, from Psalm 7:1 “O Lord my God, in you do I take refuge; save me from all my pursuers and deliver me.”
So, first we showed that Jesus Christ is Lord and God. Now the question is what is God? From Psalm 110:1 “The Lord says to my Lord: “Sit at my right hand, until I make your enemies your footstool.” We already said Lord refers to God but here David refers to at least two persons that compose God. Also, Christ says in John 10:30 “I and the Father are one.”
It appears from John 10:30 that God is one yet composed of at least two entities which are God the Father and God the Son of course. One asks now, is there any other divine entity in the composition of God? That is where the Holy Spirit is mentioned for Christ says in John 14:25-26 ““These things I have spoken to you while I am still with you. But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.”
There are more arguments to be made about why Holy Spirit is the third and final persona of God, but I leave if to Free to convince you further. That is all the time I can spare for now in writing.
Lastly, what I wanted to say is that from our mortal perspective, to attain immortality we must be saved and then resurrected at the last trump. To be saved we must repent of our sins (by acknowledging we have strayed from the way of the Father), believe the gospel (that we can only be saved through Christ) and then be sealed with the Holy Ghost (which seals our salvation).
When resurrected, we shall fully understand what God is. Until then, from a mortal’s perspective, Scripture proves that God is One and yet composed of three persona that have specific roles in our salvation.
I leave you with 1 Corinthians 13:12 “For now we see in a mirror dimly but then face to face. Now I know in part; then I shall know fully, even as I have been fully known.”