The easier way is to butterfly Christianisation of the Roman Empire. Without being propelled as an imperial cult, it would be mostly stuck in the eastern part and in a non-hegemonic position; having a too limited influence in the western part.
After this, it becomes really hard except if we can twist a bit the OP. From the moment the demographical and structural advantages goes to Christian imperial or post-imperial entities, the pressure risks to be simply too huge.
Doesn't mean you can't delay enough the process of Christianisation and ends with significant non-Christian pockets.
For something more, you'd need a more deeply structured cult, an organized religion rather than a mix of different rites (not that weren't structured themselves, but weren't an unified institutional and theological body). Not unthinkable, critically with a slower Christianisation of Central Europe.
I don't think it would be enough to prevent the conversion of these realms (critically with the advantages of such moves : diplomatic "recognition", imperial model with kingship power being reinforced, etc.). But you may end with a more or less significant pagan minority living on up to nowadays.