The previous post “Grammatical Analysis of John 1:1c and John 1:14” noted that the doctrine of the Trinity is not supported by the grammar in the opening of the Gospel of John. Since this is widely considered to be the most important and direct biblical evidence of Jesus’ preexistence and divinity, one would have to look elsewhere to establish such a doctrine. And indeed, this precisely what happened.
The doctrine of the Trinity was first developed at the Council of Nicaea in 325AD, three centuries after Jesus lived. The doctrine would be further refined in later councils of that century, but this was its formal beginning. Those determinations have since been fully embraced by Catholic and Orthodox traditions, who see the validity of the doctrine of the Trinity as an inseparable combination of Biblical teaching and church authority (e.g. the Catholic magisterium).
But many Protestants hold to the doctrine of sola scriptura, or the Bible alone. If Protestants reject the spiritual headship of the Catholic or Orthodox churches, then it must do so by rejecting the authority of those very councils that codified the Trinitarian doctrine. The Protestant is faced with two logical choices:
- Accept the authority of the Catholic church and the illegitimacy of the Protestant offshoots.
- Reject the Trinity as a 4th century innovation.
The doctrine of sola scriptura obligates the latter.
Why is it that Protestants reject nearly every doctrinal innovation made by the Catholic church, but deeply accept the corruption about the very nature of the Father and the Son themselves? If you reject the authority of the Catholic church to establish doctrines, then you should reject the doctrine of the Trinity (which is arguably of greater consequence), because it is not defensible under a sola scriptura framework.
Protestants should either become Unitarian Monotheists or else join the Catholic church.
 Directly coinciding with the formation of the magisterial Catholic church.
 This is generally simplified to mean accepting only the Bible and not the written works of later Christians outside of the accepted canon. Sola scriptura does not exclude the direct revelation of the Holy Spirit. Indeed, those who hold to the doctrine also accept that Holy Spirit speaks directly to Christians and authoritatively instructs them.
 Along with veneration of relics of martyrs, the doctrine of Mary’s sinlessness, and the real presence of the Eucharist (the Roman mass sacrifice that replaced the tithe offering and consecration).
 Not Unitarians, which is a religious group. Unitarian Monotheists are Christians who reject the doctrine of the Trinity as a largely 4th century theological innovation not found in the Bible. They are not a specific religious group.