They appeal to custom against our teaching: bad idea. Custom is often wrong.
We have two choices in the face of overwhelming sinful custom: despair or fight.
When the church persists in error for generations, God brings punishment to the 3rd and 4th generations.
Preface to the King of France, section 6
The Church is based not on externals, but on Christ's reign at the Father's right hand.
They say the Church must be visible, and the Roman bishop is its highest authority.
But what marks the Church is the pure preaching of the Word and the right administration of the Sacraments.
Hilary of Poitiers (circa 350AD) warned the Church against falling in love with buildings. Rome is doing this.
We can't judge rightness in the Church by the mainstream leadership: Aaron erred, as did the prophets opposing Elijah and Jeremiah.