apistia

This word is faith turned inside out. Where trust reaches toward God and takes him at his word, unbelief holds back or turns away. It is worth stressing that the New Testament almost never uses it for mere intellectual uncertainty, the modern idea of "doubt." It names a failure of the whole person to rely on God.

The refusal comes in several shades:

• At its hardest it is the outright rejection of God's messenger, as when Jesus' own townspeople would not receive him and "he could do no mighty work there because of their unbelief."
• A milder form is distrust of a specific promise, the wavering that Abraham refused when he was told he would father a child in old age.
• Milder still is the father's cry in Mark, "I believe; help my unbelief," where real but fragile faith names its own weak spot and asks for help.

A related word, ὀλιγόπιστος, "of little faith," is kept in Matthew for disciples who trust too little rather than not at all; full unbelief is reserved for those who reject Jesus outright. The adjective ἄπιστος names the unbelieving person, while this noun names the state itself.

Here the word shifts from "not believing" to "breaking faith." It describes disloyalty toward God, the betrayal of a bond, standing close to the idea of covenant treason. Paul uses it of Israel: some "were unfaithful," yet their unfaithfulness cannot cancel God's own faithfulness.

Hebrews sharpens this. The writer warns against "an evil heart of unbelief" that leads a person to fall away from the living God, and treats that unbelief as the twin of disobedience (ἀπείθεια). As one commentator put it, the unbelief was the root and the disobedience the fruit. This is not a mind that cannot make sense of the evidence; it is a will that will not stay loyal.