Answer:
Through interaction between traders and the locals along the trade routes.
Explanation
During their trades operation, a lot of interaction happened between the traders and the local people. During those interaction, they started to learn about each other's religion, culture, scientific development in their hometown , etc.
This is where the locals learn about the religion of the traders. After the traders left, the locals will spread the knowledge about that religion to other locals and contributed to the increasing amount of followers of that religion.
