1. assigns/accredits value to, has faith in
I'm a great believer in that
If someone is 'a great believer', 'strong believer' or 'firm believer' in something they have faith that a particular approach or thing is beneficial and it works: "I am a great believer in meditating for 10 minutes every day." If you are 'a great or strong believer in' something or a 'firm believer', you might try to champion it or encourage others to try your approach; or do things the way you believe them to work most effectively: "I'm a firm believer in using the Pomodoro technique." This is an informal expression but you will still hear it used in a wide variety of contexts.