An adherent is someone who follows and supports a belief, a religion, a political party, a method, or a leader. 'The movement has millions of adherents.' It is a formal, written word — you meet it in news reports, history books, and writing about religion.

It is not the same as 'fan' or 'supporter', and the difference is useful. A supporter backs something; an adherent has taken it on and lives by it. So we speak of adherents of a faith, adherents of a philosophy, adherents of a strict diet — things with rules you follow, not just things you like. You would not call yourself an adherent of a pop singer.

The pattern is 'an adherent of' something. 'Adherent to' turns up occasionally but 'of' is the safe choice.

There is also an adjective 'adherent', meaning sticking to a surface, but it is technical — botanists and doctors use it, ordinary writers do not. If you want the adjective, use 'sticky' or 'adhesive'.

Though actually — adherent leans slightly neutral-to-cool. Writers often reach for it when describing a group they are standing outside of and observing, rather than one they belong to. Nobody introduces themselves at a party as an adherent of anything. That distance is part of the word's flavour, and it makes adherent handy in careful, fair writing where 'follower' or 'believer' might sound loaded.