There is in fact a Noahide pledge of faith, I have it Spanish. I should try yo find it in English. Anyway it is more like vow to obey the 7 Mitzvot, not excatly a declaration of faith. It says that a Noachide can choose to reciye it in the morning or not. Opyional. Perhaps it's because it's a vow, and in case you sin, you sin double, break the mitzvah and the vow. I never rcited it. I just don't know if I would refrain form all sins.