From apistos; faithlessness, i.e. (negatively) disbelief (lack of Christian faith), or (positively) unfaithfulness (disobedience) -- unbelief.
see GREEK apistos
1. unfaithfulness, faithlessness (of persons betraying a trust): Romans 3:3 (cf. references under the word ἀπιστέω, 1).
2. want of faith, unbelief: shown in withholding belief in the divine power, Mark 16:14, or in the power and promises of God, Romans 4:20; Hebrews 3:19; in the divine mission of Jesus, Matthew 13:58; Mark 6:6; by opposition to the gospel, 1 Timothy 1:13; with the added notion of obstinacy, Romans 11:20, 23; Hebrews 3:12. contextually, weakness of faith: Matthew 17:20 (where L T Tr WH ὀλιγοπιστίαν); Mark 9:24. (In Greek writings from Hesiod and Herodotus down.)