Why did the doctrine of the trinity not come up untill hundreds of years later - what did people believe before that ?
Our understanding of divine revelation can develop. The Church grows not just in numbers, but in maturity and wisdom. Scripture tells us that the Church is Christ’s body, and we also read that Christ grew in wisdom and stature. The elements of the Trinity are all in Scripture when properly assembled and understood in concert with Sacred Tradition. Christ promised the Spirit to guide the Church in all truth until the end of this age.
Here’s an apologetic on Development of Doctrine and the “growth” of the Church. You may also want to check out Bl. John Henry Newman’s Development of Christian Doctrine or St. Vincent of Lerins’ On Development of Christian Knowledge. (links can be found here)
That is not exactly the case, it was only formally defined in the 300s, but the belief in the Divinity of Christ and the Spirit are attested to in scripture, we only had to define it when someone came along who said otherwise. It’s analogous to those “caution hot water” signs you see in bathrooms everywhere these days. The water was always hot, but we had to start putting up signs when people started to scald themselves!
It’s a complicated history, and some material heretics such as the early Modalists seem to have been attempting to be faithful Catholics and simply made a mistake on the subject before the doctrine had been formally defined.
The key thing to remember though is that when the Church defines a doctrine She is only confirming something that was already believed before. The bishops do not gather at an ecumentical council to make up new doctrines, but (with the exception of a few unusual councils like Vatican II that met for entirely different reasons) to corporately condemn new heresies that have rejected the traditional teachings of the Church.
In the case of the Trinity the word, with its modern meaning, was being used to describe God since the third century (remember the Council of Nicaea was held in the fourth century). Furthermore that term was only coined to describe the beliefs still earlier Christians had already held about God and believed was derived from the teachings of the Apostles.
The doctrine of the Trinity came up (or rather the references to It) right at the beginning in old testament times but wasn’t written down and defined until later. That is to say the doctrine wasn’t invented by the Church but the word Trinity was later applied to the revealled truth about the nature of God for the purposes of defining and teaching.
Catholics use scared tradition which coming from a Baptist background you would be missing this. Just because we didn’t put it down in writing doesn’t mean it wasn’t taught. A lot of times when herecies came about is when the Church concretely defined something.
It depends on what you mean by the “doctrine” of the trinity. Yes it is true, the Trinity itself was not formally defined by the Catholic Church until a few centuries after Christ. The Early Church Fathers (ECF’s) reveal to us the Divinity of Christ and the Holy Spirit from the very earliest of writings. The ECF’s were early Christian/Church leaders, succeeding from these men’s beliefs and writings the same Church clearly defined what she had always believed centuries later, that God exists in a Trinity.
I see that you are in RCIA Chuck, may God bless you on your journey home! The ECF writings have been a wonderful experience for me to read through, and they really have helped me reconfirm my Catholic faith. I would strongly recommend that you read through some of the writings from Ignatius of Antioch and Justin Martyr. Both men were murdered for their Catholic faith in the 2nd century, and there is a wealth of writings that they left behind that describe what they know about God, and what they know about scripture.
A simple Google search for their writings or “ECF Trinitarian writings” should provide you a wealth of quotes from their writings. If you want the entire message they left along with other ECF’s then you will want to go to newadvent.org