Chovos HaLevavos

When we speak about emunah, we say, b'emunah s'hleimah, "with perfect faith". The mitzvah is to grow in emunah, to experience a clarity, and to reach levels of deep-seated, genuine belief. There have been traditionally two different approaches to faith, one more experiential, and one more rational. However, all agree that one's emunah must run deep. The mitzvah is also constant; the Biur Halachah and others speak of "six constant mitzvos". See link.