After Christianity was made the official religion of the Roman Empire what were the immediate effects on existing pagan beliefs? Did the decline of traditional beliefs happen at different rates in Rome, Greece, and Egypt and if so where did pagan beliefs survive the longest? Furthermore how late did the last remnants of these ancient religions survive?
Do you count south asia as part of this world (see greek kings of india)? If so, then it never really declined. Hinduism continues to this day and is one of the major religions in the world.