Do I have to buy something to join?

No. The daily ritual, the reading plans, the community wall, and the first stage of progression are completely free. Email confirmation makes you a Believer with no purchase. You only ever buy a piece of apparel if you want the visible mark — the sticker is $6 and is the lowest-friction way to put the reminder in your pocket. Service, prayer, and bringing others in unlock progression past Witness; spending alone doesn't.