Faith
Belief
What you believe in your heart, and not in your head, is your faith. This is something that doesn't follow logic, it's all heart belief, it's all faith.
しんじん
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Just note that しん changes to じん.
私の母は仏教を信心しています。
My mother believes in Buddhism.
信心ぶって他人をひはんしてくるから、あの子のことあんまり好きじゃないんだよね。
I don't really like her because she sanctimoniously criticizes people.
彼の家族はとても信心深くてスピリチュアルなので、他のお酒を飲まずにスピリッツだけ飲みます。
His family is very religious and spiritual, so they don't drink any alcoholic drink other than spirits.