Horne is correct that the gospel entails more than propitiation of sins. But Horton is correct that the gospel is not something that Protestants share with the EO and RCC, because they deny sola fide and thus fall under Paul’s anathema.
I disagree with the second sentence above, but it is worth noting that such is outside the scope of my outrage. I’m not saying there is no reason to not sign the document. I’m saying the document shows a more Biblical conception of what it means to proclaim the Gospel in what it states than Horton does in his definition.
If one wants to pursue the more general issue raised in the quotation, I suggest starting with Charles Hodge, “Is the Church of Rome a Part of the Visible Church.”
PS: I left out the “not” that is in bold above. Sorry to cause confusion.