Refers to a person who has taken refuge in or converted to a religion, especially Buddhism. The term is somewhat formal and often used in religious contexts.
He is known as a Buddhist convert.
The devotee chants sutras every morning.
改宗者 refers to a convert who changes from one religion to another, whereas 帰依者 can also imply a first-time conversion or deepening of faith.
Compound of 帰依 (kie, 'taking refuge in the Buddha, Dharma, and Sangha') + 者 (sha, 'person'). The term is derived from Buddhist terminology.