Where did "they" get the idea of the Trinity if not from the Bible? All you have to do is read some of the writings of the "early church fathers" to see that their arguments for the divinity of Jesus Christ and the "triune" nature of Almighty God came from the Scriptures. Some good places to read about this subject are:
Now I will respond to your comments.
Where did "they" get the idea of the Trinity if not from the Bible?
I'm not sure what post(s) in this thread that you are referencing but at least one poster in this thread was saying a person that does not believe in the Trinity will be eternally damned. My response was that such belief is unbiblical.
All you have to do is read some of the writings of the "early church fathers" to see that their arguments for the divinity of Jesus Christ and the "triune" nature of Almighty God came from the Scriptures. Some good places to read about this subject are:
As you state, it was after scripture had closed that church fathers increasingly grappled with reconciling monotheism with God, Christ and the Holy Spirit. From this, they formalized the triune nature of God, Christ and the Holy Spirit as the Trinity. I can understand how they would come to the conclusion that the Holy Spirit is a distinct divinity, coequal and coeternal with God and Christ but I can also understand from scripture how a person might decide the Holy Spirit is more likely to be a force, or God's presence.
The body of scripture dealing with God and Christ is fairly well developed and it is clear that the Gospel writers regarded God the Father and Christ the Son as divine. Both were prayed to and both were worshiped in scripture. When it comes to the Holy Spirit, it is neither worshiped nor prayed to by the Gospel writers. Does this mean the Holy Spirit is not the third member of the Trinity? Not necessarily. But scripture dealing with the Holy Spirit is ambiguous as to its nature.
Nowhere in scripture is it written that salvation is contingent upon belief in the Trinity. God via the apostles gave us a brief expansion of his nature from what was revealed in the Old Testament but as creator of the universe, his complete nature may go far beyond even the Trinity. Therefore, believing in the Trinity may actually place limits on God.