According to Non-Trinitarians, why did God allow Trinitarianism to become the mainstream understanding of His nature? If God's nature is not adequately portrayed by Trinitarian theology, then why did God allow such an erroneous understanding of His nature to become so widespread among the members of the Church, the Bride of His Son? If God has the power and the prerogative to intervene in historical events, then why hasn't God made use of His divine privileges to ensure that the correct doctrine about His nature achieves widespread acceptance? How do Non-Trinitarians answer these questions?
(I assume that by &quot;the Church&quot; the OP includes the Roman Church and probably its Protestant daughter denominations too.)
This question implies that &quot;the Church&quot; is God's church.
The problem with this is that non-Trinitarians in general don't believe that what the question refers to as &quot;the Church&quot; is actually God's Church.
Jesus referred to the faithful as a &quot;little flock&quot; (Luke 12:32).
God's church is small:

“For many are called, but few are chosen.”
— Matthew 22:14


“Enter by the narrow gate; for wide is the gate and broad is the way that leads to destruction, and there are many who go in by it.
Because narrow is the gate and difficult is the way which leads to life, and there are few who find it. — Matthew 7:13–14

The &quot;god of this age&quot; (2Corinthians 4:4), which most of nominal Christianity follows, is not the leader of God's church.

“Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. — Matthew 7:15

That organizations calling themselves &quot;Christian&quot; happen to teach false beliefs to their members in no way means that the members of God's Church hold these beliefs.

For such are false apostles, deceitful workers, transforming themselves into apostles of Christ.
And no wonder! For Satan himself transforms himself into an angel of light.
Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, …
— 2Colossians 11:13–15

The idea that God is currently trying to save the world is almost blasphemous.
It implies that his plan is failing miserably.
Only a few, the elect, will be saved during this age.

In fact, unless that time of calamity is shortened, not a single person will survive. But it will be shortened for the sake of God’s chosen ones.
— Matthew 24:22 (NLT)

By allowing the world to deteriorate until it is almost destroyed by mankind, God sets an example of what happens when people follow Satan and decide for themselves what is right or wrong.