Wait, what? The Christianity had been spreading since the fifth-century conversion of Clovis I of the Franks. I have never heard of this Germanic assimilation and de-Christianization. Do you have any sources?
'North of the Seine' may be saying too much, but it's pretty much the going assumption that the Rhineland and South Germany was lost to the church at the time. It's where the first generation of missionaries from Britain went to and converted the natives. Quite a few lost their lives in the process if we can believe their vitae. Of course, the literary topos may be crowding out reality here, but certainly some people didn't care to have their souls saved. This, you must recall, are areas that under Roman rule were Christianised, had churches, bishops, archbishops and clerical schools. All of that disappeared along with urban living in the course of the invasions and didn't fully come back until the Pippinids started pushing missionary work into Germany.