Complex sentence: In Mexico, the Jesuits had been active in evangelizing the Indians on the northern frontier, but their main activity was in educating "criollo" (American-born Spanish) elites, many of whom themselves became Jesuits.
Split into simpler sentences: In Mexico, the Jesuits had actively evangelized the Indians on the northern frontier. But their main activity involved educating "criollo" (American-born Spanish) elites, many of whom themselves became Jesuits.