If believe was defined as "what we do," then we would be saved by "what we do/works" and not simply by believing in Christ for salvation. The greek word for believe is "pisteuo" and is defined - to have faith (in, upon, or with respect to, a person or thing), i.e. credit; by implication, to entrust (especially one's spiritual well-being to Christ):—believe(-r), commit (to trust), put in trust with.
https://www.blueletterbible.org/lexicon/g4100/kjv/tr/0-1/
The Greek words for "pistis" and "pisteuo" are two forms of the same word. "Pistis" is the noun form, "pisteuo" is the verb form. Nothing in the root meaning of either word carries any concept of works. If you believe in Christ for salvation, then you are trusting in Him to save you. Although this belief results in actions appropriate to the belief (to one degree or the other/all genuine believers are fruitful, yet not all are equally fruitful -- Matthew 13:23) the actions are NOT INHERENT in the belief.