That would be Christianity.
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
It was Chrisitanity
Through most of their history the ancient Romans had their own religion: Roman Religion. In the Later Empire Christianity spread in the Roman Empire, was endorsed by the Roman emperors and became state religion.
Christianity
Charlemagne helped spread Christianity throughout the Frankish Empire and the lands he conquered. He spread Christianity through both peaceful conversion efforts and military conquests.
No religion helped to save Roman culture. One religion spread through the Roman Empire in its later days and transformed Roman culture: Christianity.
Catholicism
What religion spread from North Africa to west Africa
There are two "Islams". The first is the religion which spread primarily through peaceful interaction or the benefits it provided under the stewardship of an Islamic State. The second Islam was an actual Empire or Islamic State which expanded through warfare and conquest. It is necessary to understand that while the Islamic Empire grew through warfare, most conversions to Islam were not compelled.
The Songhai Empire contributed to the spread of Islam and Arabic through trade and contact with Arab merchants, scholars, and traders. As the empire grew in wealth and power, it attracted Muslim scholars and traders who spread their religion and language through interactions with the local population. Additionally, the ruling elite of the Songhai Empire embraced Islam, further promoting its spread among the populace.
The Roman Empire.
Ashoka was important because he spread Buddhist religion to his empire.