Belloc offers the lucid and compelling explanation for this “bottom line” of the Church.
He might off a lucid and compelling defense in his apologetics, but he does not offer an explanation.
The explanation is that the Catholic Church considers serious disagreements with its teachings to be heresy.
Why not just call serious disagreements, serious disagreements?
Because the word "heresy" invokes canon law. And that means that the Church makes a presumption of jurisdictional authority over all of humanity, and thereby strips people of the free will necessary to agree with and join the Church, or not.
This is simple logic. Law has no power outside of its jurisdiction. Therefore "heresy" can only truly be applied to Catholics who are under canon law, who seriously disagree with Church teachings.
But Catholics regularly tell non-Catholics that they are committing heresy by seriously disagreeing with Catholic teachings.
So either Catholics are wrong in their extension of the concept of heresy outside of the Church, or the Church actually presumes authority over the whole of humanity, whether humanity accepts the Church or not.
I wonder if Belloc mentioned any of this.
I doubt it.