Faith, Belief and Pistis, πίστις
The first issue is whether the word πίστεως means belief, faith or faithfulness. The word may have all of those meanings, but what do the dictionaries say?
BDAG, 3rd: “1. faithfulness, reliability, fidelity, commitment” Anyone who has the slightest bit of doubt should go and inspect Additional Greek Lexicons in a photo archive provided with complete source information.
The meaning is supported by the same word in different parts of speech, i.e. πιστός means faithful (BDAG). And the verb πιστεύω means to have fidelity, be faithful, be trustingly faithful. Thayer supplies, to commit oneself trustfully, and BDAG suggests the meaning commitment. Even though the Christian Lexicons are very reluctant, and almost loathe to unify, or note that the word group is unified, the sense is obviously, be faithful, faithful, and faithfulness for the verb, the adjective, and the noun form respectively.