Tithing is definitely a thing in a lot of the major religions, at least. Just how mandatory it is varies by region, denomination, time period, etc., but I imagine there's almost always at least some non negligible social pressure to pay tithes.
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And if society forces that on God's behalf then that's immoral, even for a religious Libertarian who already sacrifices for their God and religion. So long as your tithe isn't going to some other guy a Libertarian should be fine with it.