The whole scoop is covered well in JSTOR's The Catholic Revival in Osnabruck and Minden, 1591-16511.
Long story short:
- It was the Prince Bishopric of Osnabruck, which while including the spiritual responsibility of the (larger) See of Osnabruck, was a secular post elected by the 24 members of the cathedral chapter (canons and deacons, etc.) of Osnabruck. Seeking to convert northern Westphalia back to Roman Catholicism, Popes in the final decade of the 16th century began more aggressively to exercise their right to unilaterally fill any vacancies occurring in odd-numbered months.
In the mid 1610's this swung the Osnabruck cathedral chapter to a Catholic majority, which then sought out, in succession, two energetic Counter Reformation leaders meeting the requirement of being "feudal nobility". Recall that Von Wartenberg met the latter as the son, by a morganatic marriage so ineligible to be heir, of Elector Maximillian of Bavaria.
Note that the theological (as opposed to liturgical) qualifications of von Wartenberg were impeccable: He had completed his education at Collegium Germanicum in Rome, seen to be training German Counter Reformation leaders; served as Chancellor for the Prince Elector (Archbishop) of Cologne; and assisted the Archbishop at the Diet of Ratisbon that stripped the lands and Electoral dignity from Frederick V,
erstwhile Elector Palatine of the Rhine, to Maximillian of Bavaria.
See also Diocese of Osnabruck in New Advent.
- William C. Schrader, "The Catholic revival in Osnabrück and Minden, 1591-1651", The Catholic Historical Review, Vol. 78, No. 1, Jan. 1992, pp. 35-50.