John 3:6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
John 4:24 God is a Spirit: and they that worship him must worship him in spirit and in truth.
1 Timothy 2:5 For there is one God, and one mediator between God and men, the man Christ Jesus;
The trinity doctrine contradicts those verses. That is why churches that teach the trinity discourage studying the bible, declaring that church tradition is of higher authority than the word of God.
The only importance is that the Roman Catholic Church was the sole defender of the faith for over a thousand years. A great many people learned everything they know from the Catholics, even if they grew up in a protestant church. Protestant churches don't teach the bible, and that's why a lot of Christians don't know the first fact about what they claim to believe.