Start a cleric at level 1 and have his (or her) deity be the local village totem/spirit/ancestor. As long as he never leaves the area, praying to Althora, the local waterfall spirit grants him all the spells he needs.
Once he leaves his village, he’s going to need a stronger god with more believers. Spells might come sporadically or at lower strength. Time for proselytizing! He’ll need to figure out why the fishermen and their families in Big-City-on-the-River should pray to her: “The goddess Athora is the protector of all those who fish the rivers.”
As his level increases, his ability to impress and convert would also grow. Eventually, Althora might be the goddess of weathering change. She would be known, if not worshipped, anywhere on continent or in the world, so that the cleric call on her from anywhere. By then, the PC would be the founder of a cult, sect or major religion. The waterfall back home would be a site of pilgramage, where his has established his abby or cathedral. Someday he could be a saint or perhaps a martyr!
This gives the player a chance to make up the trappings and lore of his character’s religion, make visible impact in the campaign and is a terrific opportunity to develop plot hooks.