believe in

1 :  to have faith or confidence in the existence of (something) Do you believe in ghosts?

2 :  to have trust in the goodness or value of (something) She believes in (the value of) regular exercise. I believe in working hard to achieve success. She doesn't believe in using pesticides.

3 :  to have trust in the goodness or ability of (someone) Despite his problems, his parents still believe in him.