For the particular case of [Iceland](https://en.m.wikipedia.org/wiki/History_of_Christianity_in_Iceland), the whole thing was decided in on legal grounds. Parliament (called “Althing” in Iceland), at their meeting in either the year 999 or 1000, appointed the pagan law speaker (sometimes translates a “lawyer”) [Thorgeir Thorkelsson](https://en.m.wikipedia.org/wiki/Thorgeir_Ljosvetningagodi) to settle the matter. Facing unrest between pagans and Christians, Thorgeir (a pagan) lay under a blanket for a day before deciding in favor of Christianity. Pagan sacrifice was allowed to continue in secret, provided that no witnesses could be produced to it.
The wiki page quotes a historian as saying this account is unbelievable, but the only one they have. Certainly a very interesting chapter in history.
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