Religion has always existed. Atheism is more recent. No natural society ever existed whose members did not believe in the supernatural.
The question, then, could be how a particular religion spread.See the Related Link.
Religion likely originated as a way to explain fundamental things people were not able to explain, such as their existence, how the planet was formed, and why the universe appears the way it does.
Many religious texts contain references to eternal salvation and/or punishment, claiming those who do not follow the religion cannot be "saved". This compelled people to spread their religion in what they probably viewed as an honorable attempt to "save" people they knew.
One of the ways religion spread was through merchants and travelers who brought the messages during their journey. Religion also spread through worshipers. In short migration is responsible for the spread of religion because as people traveled they tend to maintain their religion
Religions have spread through migration, through conquest, and through missionary work.
Universal religions
Most Roman, Persian, and Greek religions spread around the silk roads during that time.
Started Buddhism.
islam
Hinduiasm
Chinese religions often spread throughout the beliefs of the family members.
Bridging World History - 2004 The Spread of Religions 1-7 was released on: USA: 2004
They do spread, but because we are not informed of their expansion we are ignorant of their growth. It is this writers opinion that most increase by 1/2% to 2% per year. This may also included those who are born into that religion and not by conversion.
The same way other religions spread, through preachers,teachers, missionaries. How do I know?, because I am one.
This is something that happens practically to any religion.
The spread of world religions.
There are a few religions that spread through missionaries. The main religion would be Buddhists.