For some people, "faith" means unreasonable belief (or at least belief apart from logic, for example, the book I just read).
The word "faith" is used twice ("faithful" or "faithfully" are also used a lot) in the Old Testament (KJV). The Hebrew word is "emuwn" (ay-moon) or "emuwnah". It implies "established", "trustworthiness", "security", "fidelity", or "firmness".
The word in the New Testament is "pistis" (and is used more times than I care to count, over 200 - although it is sometimes "little faith"). Pistis implies "reliance", "constancy", "assurance", "conviction", or "trust".
Ok, so there is me and God involved in this faith. Which one of us is "trustworthy", "reliable" or "constant"?
So, our faith is in the trustworthiness, constancy, and fidelity of God.
If our faith is lacking, there are elements of us accusing God of lacking these attributes. So either we are blaspheming God (demeaning His nature) or we are creating an idol (a god without the true attributes of God).
Now we are engaged in an internal conflict with sin (Galatians 5:17). So, it is not unusual that we will sometimes stumble (1 John 1:8). But if we are continuing in sin, we are not saved (1 John 1:6).