Complex sentence: Poland's first historically documented ruler, Mieszko I, was baptized in 966, adopting Catholicism as the nation's new official religion, to which the bulk of the population converted in the course of the next few centuries.
Split into simpler sentences: Poland's first historically documented ruler, Mieszko I, accepted baptism in 966 and adopted Catholicism as the new official religion of his subjects. The bulk of the population converted in the course of the next few centuries.