“Who is Jesus” was a hotly debated topic in the early Church. Some said a man who was very in tune with God, others said God who appeared to be a man but wasn’t, and others said partly man and partly God wrapped together. If Jesus isn’t fully God, as all of these indicate, then is he part of the eternal Trinity? Certainly not. Today, many Christians assume understanding Jesus as part of the Trinity and fully God and fully man is obvious from reading the Bible. I argue it’s only obvious because we approach the Bible with these conclusions; when we study the process it took to define the Trinity and Jesus human / divine relationship we see that Scripture was used by all sides and coming to a conclusion required centuries of debate capped with an authoritative decree that today all Christians trust.
Arius was born in 250 A.D. and was a priest in the church of Alexandria, Egypt. He reasoned from Scripture that Jesus was part of creation. If Jesus was created then he was less than and under God. Certainly he wasn’t part of a Trinity. He couldn’t be considered divine. Worse still, as part of creation Jesus would decay and succumb to corruption. Does this sound like the Jesus we know today? Not at all, but the idea caught and spread like wildfire. It was “the” debate of the early Church and both sides built their belief upon the foundation of Scripture. How was a decision reached?
In 325 the Council of Nicaea was called to settle this matter. From it we get (the first version of) the Nicene Creed. The older Apostles’ Creed was very focused on the humanity of Jesus (it was written primarily to refute Gnostics) but in contrast the Nicene Creed focuses very much on Jesus’ divinity. Further, at the Council of Chalcedon in 451 the relationship between Jesus humanity and divinity was defined; he is 100% of both natures in a perfect union.
This history lesson is important because I’ve heard it said many times that “the Trinity is in the Bible” and that’s true—the Father, Son, and Spirit appear together at Jesus’ baptism for example—but does Scripture tell us whether the Spirit proceeds from the Father and the Son or just the Father? This is a major sticking point between the Greek Orthodox and the Roman Catholic Church today (look up “filioque” for more on that). Scripture was used and still is used to argue both positions.
The same can be said of Jesus. Is he a man with a separate divine consciousness? Is he not really a man but God in the appearance of man? Is he a very wise man with a close relationship with God but not really God? All of these, and more, were proposed and defended from Scripture.
The process for understanding the Trinity, Jesus’ hypostatic union, the list of books in the New Testament, whether gentiles need to be circumcised to be Christian, whether Mary was assumed into heaven, and our other doctrines went through this same process of serious study of Scripture, evaluation of the apostles’ teaching, strenuous debates, and an ultimate binding decision all overseen by the loving guidance of a patient Holy Spirit.
This is yet another reason Catholics believe the Holy Spirit guides the Church to teach infallibly.