When was the time when we can say pagan religions in Europe have been completely supplanted by Christianity?
Can we see remnants of pagan religions in Europe by the Late Middle Ages or in the Renaissance? Like there's a village in a remote area still worshipping a pagan god or something like that.
The term pagan is somewhat unhelpful because it can refer to pretty much all religions that are not Abrahamic. Christianization was not one big push to make the whole of Europe Christian. It was instead a series of slow conversions done in chunks that was never fully complete. By the 9th century Western Germany had been Christianized but the areas around and east of the Elbe remained as Slavic pagans until the baptism of Mieszko I in 966. Even then the areas around Brandenburg and Pomerania would remain Slavic until the beginning of the 13th Century.
Scandinavia remained Norse until the early 11th Century were they were slowly Christianized. However local holdouts would remain until the mid 12 century. However Finalnd was not Christianized fully until the 14th century and the northern Sami peoples would remain unchristianized until the 18th Century.
Although many Slavs would be Christianized under Orthodoxy in the 9th Century with Vladimir however the areas of Ukraine and the steppes were not yet inhabited Slavs and remained Tengri until the 13th Century were they would convert to Sunni Islam. The Tatars who inhabited this area would not be converted but instead slowly pushed out by Russian colonists.
There was also Lithuania, a Romuva state which held out in the area of modern Lithuania until the 13th century were it conquered the areas of Belarus, Ukraine, and Western Russia forming a massive pagan empire. They converted in the early 15 Century for political reason as they were threatened by the Catholic Livonians, Poles, and Teutons. They still however practiced pagan traditions in private.
In short Europe was never totally Christianized. There was always a Pagan village somewhere. But in practical terms there were no pagan powers by the 15th Century with the Grand Duchy of Lithuania being the last.
Sources Richard Frucht "Eastern Europe [3 volumes]: An Introduction to the People, Lands, and Culture" Dr. Sæbjørg "The Christianization of Norway "
Early Medieval popes often incorporated pagan festivals and ideas into Christian theology and Christian celebrations in a way to convert more pagans by easing the transition.