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?
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Christianity didn't really begin to crowd out the other religions in Europe until after Constantine and Julian's reigns in the Roman Empire, although well before that time (the 190s CE) the Empire had began an Imperial cult in which the Emperor was revered as a god, which probably resulted in a decline of the older religions. Per Wikipedia, Theodosius I officially stopped support of non-Christian cults in 391, although Rome itself fell only around 85 years later (it was a gradual fall), so I don't know that that would have completely quashed the old school Roman religion centering around Jupiter and Mars and so on that I think you're interested in.
The Roman religion was, of course, just the Greek religion with different names, so I think to an extent the one explains the other (I'm not too familiar with Greek history after Roman dominance anyway, to be honest)
The "Norse" religion, by which I'm guessing you mean the one where they worshiped Thor and Odin and Freya and so on, was as much a Germanic religion as a Scandinavian one; I think we couple it with the Scandinavians today because that was the version that survived the latest. A lot of what we know about that religion comes from stuff that was written down about it shortly after areas were Christianized, and this apparently was not done in Iceland, for instance, until the 13th century. Otherwise, I believe the Scandinavian countries converted to Christianity between 1000 and 1200. This was generally done by the rulers converting first or having already been Christian before becoming King or the equivalent, and then ordering their people to adopt the religion. The Marches to the east of Germany remained pagan for quite some time as well; if memory serves the Order of the Teutonic Knights was created to push these people back and/or forcibly convert them to Christianity.