And even they, if they do not continue in their unbelief, will be grafted in, for God has the power to graft them in again.
Romans 11:23 ESV

The word for “unbelief” is apistia (απιστια), which can also mean faithlessness, such as in Romans 3:3. In this instance, even if “unbelief” is the intended meaning, it is unbelief which leads to disobedience. In the context of salvation, the two ideas are inextricable.

Nobody is saved by assenting to a fact, but by believing in the character (name) of YHWH to forgive and keep his promises, which belief will always lead to faithfulness. In Scriptural parlance, faith and faithfulness are not necessarily the same thing, but there is no separating faith (or belief) from faithfulness, nor unbelief from unfaithfulness.