What was the major contribution of emperor Constantine?

Emperor Constantine's major contribution was the establishment of Christianity as a prominent religion in the Roman Empire. He issued the Edict of Milan in 313 AD, which granted religious tolerance to Christians and ended their persecution. Additionally, he convened the First Council of Nicaea in 325 AD, which sought to address theological disputes and unify Christian doctrine. Constantine also founded the city of Constantinople, which became a vital center for Christianity and the Byzantine Empire.