The doctrine of the Trinity is central to Christianity. It holds that one completely divine God exists in three persons, the Father, the Son, and the Holy Spirit, who are distinct yet consist of the same essence or substance. They are distinct from each other in their relations to one another and in the way they relate to creation. All things are from the Father, through the Son, and in the Holy Spirit.
Jesus Christ lived, beginning his ministry around age 30, was crucified, and as Christians believe, was resurrected 3 days later.
Christianity immediately began to spread through the Roman Empire and beyond through the Twelve Apostles, other followers of Jesus, and those they recruited. For about a generation, Christian belief was transmitted orally. Beginning around 65 CE, the Gospels were recorded. Oral traditions and rituals continued to be established. As Christianity gained in popularity, it began to face opposition from the surrounding Jewish and Greco-Roman culture.
The theologians who had direct contact with the Apostles are referred to nowadays as the Apostolic Fathers. Tradition continued to be established, but the writings of the Apostolic Fathers did not find their way into the New Testament. During this time, Christianity faced increasing persecution. Prominent Christians were martyred. Christianity began to distinguish itself more from Judaism and struggles to define itself theologically.
The Council of Nicaea confirms that the Son of the Trinity is equal in substance to the Father. The Son is truly, fully God.
The Holy Spirit was confirmed as equal to the Father and the Son.
“Trinitarian theology, in so far as it is concerned with what ‘kind’ of God Christians worship, is far from being a luxury indulged in solely by remote and ineffectual dons; it is of cardinal importance for spirituality and liturgy, for ethics, for the whole of Christian self-understanding.” (Rowan Williams, Wrestling with Angels: Conversations in Modern Theology, p. 142)