When did ancient religions start to experience dropping numbers of adherents in Europe?

Question

I am talking about ancient Greek, Roman and Norwegian religions. When did ancient religions start to lose ground to modern religions such as Catholicism in Europe?

Answer 1

About the 3rd century C.E. This was termed the "Age of Anxiety" by E.R. Dodds. The latter covers the cooptation of some pagan practices into the Cristian tradition. The spread of Christianity and the popularization of the doctrine occurs later, around Charlemagne and the marriage of the political and the religious in the person of the "Holy Roman Emperor". Hope this helps.