Final answer:
Constantine changed the religious policy of the Roman Empire by issuing the Edict of Milan in 313, granting religious toleration and legalizing Christianity. His conversion to Christianity resulted in its rapid expansion and the decline of traditional Roman polytheism. Constantine's support implicitly positioned Christianity for its eventual elevation as the official religion of the empire under Theodosius.
Step-by-step explanation:
Emperor Constantine significantly changed the religious policy of the Roman Empire by issuing the Edict of Milan in 313, which granted religious toleration across the empire, particularly allowing Christianity to be practiced legally. Christianity was not made the official religion of the empire at that time—that occurred under Emperor Theodosius at the end of the fourth century. However, Constantine's conversion to Christianity and subsequent support led to its expansion throughout the empire and the end of state-sanctioned persecution of Christians. By aligning himself with Christianity, Constantine transformed the church's status, increased its institutions' wealth, and entwined Christian identity with secular power.
Christianity's new status profoundly impacted the empire's religious landscape. The influx of new members to the Christian Church after Constantine's conversion shifted the religion's demographic from common people to also include Roman elites who wished to remain in favor with the emperor. With the decline of traditional Roman polytheism, the path was set for Christianity to become the dominant faith of the empire, culminating in Theodosius declaring it the official religion.
Constantine's policies marked the beginning of the transformation of the Roman Empire into a Christian state, eventually influencing the abandonment of traditional Roman gods by the populace and the elite. This initiated the era of the state church of the Roman Empire, laying the groundwork for its eastward shift and the political and social landscape of Byzantium.