Should mainline Catholic and Protestant churches ever unite properly, it’s going to affect Europe which was the most Christian continent in history. In recent memory that’ll go to Africa instead but one could argue that it’s due to colonisation. Even then that’ll affect Africa on some scale. The European countries most likely to be heavily affected by this are Italy and Germany.
Both of them were allies in WWII, forming the Axis Powers with Japan. Italy’s where the Vatican’s in and Germany’s the birthplace of Protestantism via Martin Luther. Should this ever occur it’s going to horrify some followers so badly they’d move to other religious sects instead. Scandinavia and the Lowlands would be badly affected. Even Muslim countries would be affected.
Like with their Christian counterparts, some would jump to more mystical sects instead. Ireland itself would be affected in a way that is by having some people rediscover its homegrown medieval insular Christianity and its medieval Gaelic culture in general. That’ll pose a big threat to the big continental Church like with Sufism and Evangelicalism.
If these three refuse to assimilate into that church, they’d work with the Orthodox and Apostolic branches who also share their sentiments.