How does a religion become official?

For anything to become "official," it has to be codified by statute, or at least by general agreement of a group.

Many religions that are considered "official" got that way via political or social organizations that formed within the religion. Others came about due to conversions of existing rulers. Often they are exported to other areas by force of arms. Conquerors, who often use the excuse of pillaging in the name of some god, tend to being their religions with them.

In Western Europe some religious groups achieved political control from within (or, in the case of King Henry the 8th, changed affiliations).