Which of the following actions was taken by Julian the Apostate?

Julian the Apostate, known for his efforts to revive paganism and diminish Christianity during his reign as Roman Emperor from 361 to 363, did indeed allow heretical bishops to return to the church. This decision was part of his broader strategy to diminish the influence of Christianity, which he viewed as a rival to traditional Roman religions. By permitting heretical bishops to return, he aimed to create divisions within the Christian community, undermining its unity and strength. This action is significant as it highlights Julian's commitment to reinstating paganism and reflects his belief in a pluralistic society where various religious practices could coexist, albeit with a primary focus on reviving pagan rituals and institutions.

In contrast, other options related to actions such as abolishing pagan schools, establishing a Christian liturgy, or building churches for Christians do not align with Julian's known policies and actions during his rule. His tenure was characterized by an active opposition to the expansion and institutional power of Christianity rather than promoting its practices or institutions.