By definition all Christians ate TrinitArian.
Sorry, no.
ALL Christians haven't adopted the pagan trinity. The cult originating with Simon Magus (Catholicism) acquired that belief from paganism.
The Surprising Origins of the Trinity Doctrine
You can keep babbling about Roman Catholics if you like. But non trinitarianism is exclusive to Roman Catholic derived, that is western, heretics. You don’t find Unitarianism in the Eastern, or Indian, or Armenian, or the ancient Syriac or Coptic or Assyrian or Absynian churches.
You know...the ones who were never under Roman Catholic dominion. They are exclusively Trinitarian.
Funny that.
Religion, like politics, sometimes stirs the worst kinds of passions in us. But I hope those Christians posting here will not give in to their irritation or anger with those who believe differently so that personal insults take over the discussion.
As for the Roman Catholics, their doctrinal claim spanning millenia is that they trace their origin back to the Apostle Peter, the disciple upon which Jesus said he would build his Church, and therefore he (Peter) was the first pope.
And the Catholic catechisms definitely include teaching of the Trinity.
The mystery of the Holy Trinity is the most fundamental of our faith. On it everything else depends and from it everything else derives. Hence the Churchs constant concern to safeguard the revealed truth that God is One in nature and Three in Persons.
Catholic Doctrine on the Holy Trinity
The catechism on the Trinity:
The Trinity – Catechism of the Catholic Church :: Catholic News Agency
And no, I am not nor have I ever been Roman Catholic.
In teaching the History of the Christian Church and the development of Christian thought, however, quite a bit of time is spent on the various concepts of the Trinity and some of the worst and most destructive church fights in history have been over that single issue.