I've always found it interesting that the Reformation 'began' in Norther Europe. It strikes me that most of the various Christian sects seemed to form within different ethnic groups. So, I'm wondering how long Catholicism lasted in northern Europe before being official rejected.
The last major pagan group in Europe was the Sami in northern Scandinavia.
Although missionaries traveled north and churches were built aready in the 16th-17th century, the sami were predominantly pagan until forced christianization that started in the 18th century. (1720 in Norway, late 18th century in Sweden).
Although officially Christian since the 18th century, the Sami didn't really take to christianity until the 19th century with Lars Levi Laestadius' revival movement.
An answer to the question in your title would be the 14th/15th century and relate to the Duchy of Lithuania. For an account of the causes see eg. W Urban: The Conversion of Lithuania (1987).
It could be that the question is really about the end of Catholicism is Northern Europe. If so my answer is this:
- Catholicism was not rejected all over Northern Europe. Lithuania is in Northern Europe, and it is still a very Catholic country. .