Complex sentence: In Europe there was no split between Old Order Amish and Amish Mennonites, all congregation became in a way "Amish Mennonites" and assimilated into the Mennonite mainstream in the second half of the 19th and the first half of the 20th centuries.
Split into simpler sentences: In Europe there was no split between Old Order Amish and Amish Mennonites. Congregations initially converged into "Amish Mennonites" and assimilated into the Mennonite mainstream in the second half of the 19th and the first half of the 20th centuries.