noun, noun or participle which takes 'suru', intransitive verb
religious conversion
Refers to changing one's religion or denomination. Often used with する to mean 'convert to a religion'.
彼はキリスト教に改宗した。
He converted to Christianity.
改宗の理由は人それぞれだ。
The reasons for religious conversion vary from person to person.
転向 is a broader term for changing one's ideological or political stance, while 改宗 is specifically about religious conversion.
Compound of 改 (change) and 宗 (religion/sect). The exact historical derivation is uncertain, but the kanji combination transparently conveys 'changing one's religious affiliation'.