“But who has the authority to declare what are the core beliefs? Furthermore, since it is claimed that Protestants, despite the large number of denominations, are actually united in core beliefs, what is the justification from cutting off and separating from other Christians over minor issues?”
Been decided for a few hundred years - even longer, if you go back to the early creeds.
We don’t separate over minor issues, except to acknowledge liberty among the rites.
“In Essentials Unity, In Non-Essentials Liberty, In All Things Charity”
That does not answer my question of who has the authority to decide. If such an authority did exist in the church where is it today? Where was that authority in the 16th century when the disputes over the Reformation arose?