“Celtic Christianity spread from…
“Celtic Christianity spread from Ireland”
91.6% Catholic (not counting Ulster)
“to, Britain,”
High Church Protestant with a substantial Catholic minority (their Census doesn’t seem to take denominational figures for England and Wales)
“Iceland,”
87.1% Evangelical Lutheran
“North and Central France,”
83%-88% Catholic
“Belgium,”
75% Catholic
“Holland,”
31% Catholic, 21% Protestant, 40% unaffiliated. Mostly Reformed Protestant in the past.
“Germany,”
34% Protestant (mostly Lutheran), 34% Roman Catholic
“Southern Denmark,”
95% Evangelical Lutheran
“Czech Republic,”
39.2% Catholic, 4.6% Protestant, 39.8% atheist
“Switzerland,”
The home of Geneva, yes, but 46.1% Catholic, 40% Protestant
“Northern Italy”
Predominantly Roman Catholic (they don’t seem to keep figures)
“and Slovenia.”
70.8% Roman Catholic (2% Uniate)
Forgive me if I’m being dense, but I don’t see any overwhelming trend towards predominant Protestantism here. Of course, a large number of the countries that did turn out to be majority Protestant are in this sphere; but so, too, are a large number of the countries that turned out to be majority Catholic. (If you go much further east, you end up in the Byzantine and Turkish sphere of influence; if you go much further west, you fall into the ocean.)