Celtic Culture 'Converted'

By 50 BC, the Romans were ascendent.  Celtic culture, especially in the south, came under dominion of the empire.  The map immediately below shows the sea and river routes by which the Romans spread to the north.

This map shows the furthest limit of the Roman Empire around 200 AD, along with urban settlements of 'Colonia/Municipium', smaller estate development 'Siedlung,' 'Kastell' or castles and Villa rustic.  The outer limit of Roman empire was termed the 'Limes' line.

https://en.wikipedia.org/wiki/Limes

Christianity slowly overtook both the Roman Empire, and also  formerly Celtic druid areas.
The limestone cross below was found in Aargau, Switzerland and is from about 600 AD.

This marble column from about 800 AD is from the Muestair Cloister in Graubunden Switzerland and  it  seems to  incorporate  Celtic design into what was a Christian monastery founded by Charlemagne.