Maybe one can look at what happened in the USSRor her succcessor states. Several uralic or turkic native religions experienced a popular resurgence in answer to the detoriating authority of the state. Nationalism was a key point to distance the believers from the russian and atheist elites of this time. Naturally the orthodox church experienced a likewise revival in all demographic layers after the fall of the USSR.
Therefore the main problem is, that with Christianity you have a ideology with which the multitude of people are quite content with and gives quite convincing answers to the big questions of life, for that time. Unless you give the believers a reason to question the authority or the use of any particular ideology, leading maybe to rebellions like the Reformation, they will stay under the umbrella of this religion/ideology. Additionally it is quite hard to embrace another religion in the 17th century, since this would mean you would leave the community you are born into. It would be a social stigma and any new religious movement needs to build a community in the underground first, to reach a critical mass to convince others into joining their cause.
So, give them a reason to deeply distrust the form of Christianity they are currently adhering to and give them a viable alternative. The poor and already stigmatized social strata are a particular good breeding ground for this kind of revolution: Make 'neopaganism' particularly trustful and rebellious against the 'unjust and filthy rich' state, defined by its christian nature, and let it have institutions, working in the underground, to care for the poor. Maybe you get a widespread movement rallying around neopaganism as an alternative to Christianity and to break with the unfair traditions of the past, which led to the current conditions. (see also: the Dalit Buddhist movement in India, which tries something similar)
A POD might be the 30 years war. Germany was particularly hard-hit and parts of the landscape were pratically depopulated. Maybe some lunatic/charismatic preacher propagating the rejection of warmongering Christianity presents an alternative which is clearly better and may be considered 'pagan', since it radically rejects traditional christian doctrine and rituals. But I cannot imagine, that something completely resembling the paganism of ancient times arises, since much knowledge of the practices was lost and such a movement, if it comes from 'bottom up' will probably rely on Christianity as an inspiration nevertheless. Maybe some cult of popular saints as 'gods' with an apparantly monotheistic godhead may do the trick, as said before. (Even the Santa Muerte cult, basically revival of mexican religious practice, relies heavily on catholic imagery and rituals.) With emerging romanticism this cult may take a more 'pagan' look.
Therefore the main problem is, that with Christianity you have a ideology with which the multitude of people are quite content with and gives quite convincing answers to the big questions of life, for that time. Unless you give the believers a reason to question the authority or the use of any particular ideology, leading maybe to rebellions like the Reformation, they will stay under the umbrella of this religion/ideology. Additionally it is quite hard to embrace another religion in the 17th century, since this would mean you would leave the community you are born into. It would be a social stigma and any new religious movement needs to build a community in the underground first, to reach a critical mass to convince others into joining their cause.
So, give them a reason to deeply distrust the form of Christianity they are currently adhering to and give them a viable alternative. The poor and already stigmatized social strata are a particular good breeding ground for this kind of revolution: Make 'neopaganism' particularly trustful and rebellious against the 'unjust and filthy rich' state, defined by its christian nature, and let it have institutions, working in the underground, to care for the poor. Maybe you get a widespread movement rallying around neopaganism as an alternative to Christianity and to break with the unfair traditions of the past, which led to the current conditions. (see also: the Dalit Buddhist movement in India, which tries something similar)
A POD might be the 30 years war. Germany was particularly hard-hit and parts of the landscape were pratically depopulated. Maybe some lunatic/charismatic preacher propagating the rejection of warmongering Christianity presents an alternative which is clearly better and may be considered 'pagan', since it radically rejects traditional christian doctrine and rituals. But I cannot imagine, that something completely resembling the paganism of ancient times arises, since much knowledge of the practices was lost and such a movement, if it comes from 'bottom up' will probably rely on Christianity as an inspiration nevertheless. Maybe some cult of popular saints as 'gods' with an apparantly monotheistic godhead may do the trick, as said before. (Even the Santa Muerte cult, basically revival of mexican religious practice, relies heavily on catholic imagery and rituals.) With emerging romanticism this cult may take a more 'pagan' look.