信仰(xìn yǎng) - faith, belief, conviction in Mandarin

信仰 (xìnyǎng) is faith of the deep, life-organising kind: religion, political conviction, a principle someone would not trade away. It is a noun first and a verb second, and it belongs to serious or formal register.

Do not use it for ordinary believing. "I believe you" is 我相信你 (wǒ xiāngxìn nǐ); "I think it will rain" is 我觉得 (wǒ juéde). Reserve 信仰 for the things a person builds a life around.

For saying you follow a particular religion, the natural verb is plain 信: 他信佛 (tā xìn Fó, he is a Buddhist), 她信基督教 (tā xìn Jīdūjiào, she is a Christian). 信仰 then names the thing itself: 宗教信仰 (zōngjiào xìnyǎng, religious faith).