Discuss the relationship between religion and empire by comparing and contrasting the relationship between Christianity and the Roman Empire and Islam and the caliphate.

Christianity was spread and protected by the Roman Emperors through their conversion to Christianity and through the favors they granted Christians; similarly, Islam was spread by the caliphate, but it was initially spread by military conquest rather than political decree (as Christianity was). Both Christianity and Islam promised salvation in the afterlife and worldly advancement to their adherents.
During the first centuries CE, Christians were persecuted in the Roman Empire. Though these waves of persecution waxed and waned, Christians were generally regarded with suspicion by pagans in the empire and were seen as traitorous for not supporting the semi-divine nature of the emperor. However, there were lulls in the persecution of Christians, and the Roman Empire tolerated other religions, such as Mithraism from Persia.
After Emperor Constantine made Christianity legal in 313 CE, Christianity became more tolerated in the Roman Empire, though Constantine himself continued pagan traditions such as building statues to pagan gods. His Edict of Milan in 313 returned lands that had been confiscated to the Christian church, and his favoring of Christians in granting government positions helped convince people in his empire to convert. In addition, during his reign, Sunday was recognized as the day of rest for everyone save agricultural workers.
Christianity was still persecuted by emperors such as Julian in the 360s, but it gained adherents over the next several centuries with its universal promise of salvation and its paths to worldly advancement within the church hierarchy. In addition, the emperors following Constantine were responsible for defining the orthodox rules of the religion and making sure that their subjects upheld this orthodoxy. In 380 CE, Nicene Christianity became the official religion of the Roman Empire under Emperor Theodosius I, and paganism was outlawed in 391. Cults such as Mithraism continued to attract followers, but Christianity eventually all but eclipsed paganism through the close connection between the Roman Empire and the Catholic Church, which was based in Rome.
Like Christianity, Islam spread through the support of political leaders. After Muhammed's death in 632 CE, Islam spread under the four caliphs who followed him until 655. Unlike Christianity, Islam spread through military conquest. Starting in 634, Muslim armies mounted incursions against the Sasanian empire to the east and the Byzantine empire in Palestine and Syria. During the Umayyad dynasty (661 – 750), Muslims developed their navies, and in the 8th century, Muslim armies conquered lands east of the Arabian Peninsula along the Indus River and in northern Africa and Spain. Muslim armies were successful in part because the lands they conquered, such as Spain, had been riven by internal conflicts.
People in conquered lands began to convert to Islam in part because of its promise of salvation and also because it offered a more stable political situation and the end to internal divisions. Over time, under the Abbasids (750–1258), non-Muslims under the caliphate, including Jews, Christians, and Zoroastrians, were penalized with a tax if they did not convert to Islam. They were not, however, forced to convert. Many chose to convert because they could become part of a greater political and spiritual entity that was well run and they could still retain a connection to their communities and local cultures while practicing as Muslims. The reasons that Christians and Muslims converted over time were similar, as both religions promised salvation and material and political advancement.