Unfortunately the evidence some 200 years later shows he had a good point... the Protestant countries came much much sooner to a republic.
Unless you don’t count the Polish/Lithuanian Confederation, which was a Republic with an elected king run by a Senate or the Venetian Republic.
Except, of course, that Adams and the Framers looked to Switzerland as an important example of a successful republic. And Switzerland was an entirely Catholic country for the first three centuries of its republican system.
And France had a republic before any of the major Protestant European countries, with the exception of Holland.