"Yo yacchraddhah sa eva sah" — whatever faith a person holds, that is what they are. This is one of the most quoted lines in the Gita, and for good reason. Krishna is saying something radical: a human being is defined by their faith.
Water takes the shape of whatever vessel holds it. Faith works the same way — it molds itself to a person's inner nature. A sattvic person is drawn to deities and wisdom. A rajasic person reaches for power and pleasure. A tamasic person drifts toward darkness and inertia.
The practical implication is clear. To elevate one's faith, one must refine one's inner nature. Faith does not change from the outside in. It changes from the inside out.