The Greek verb apisteo means to disbelieve or to be unfaithful/untrustworthy. It combines a (negative prefix) and pisteo (to believe/trust), making it the precise opposite of faith. It appears about 8 times in the New Testament.
Apisteo is used in some of the most sobering passages in the New Testament about unbelief. After the resurrection, Jesus appears to the eleven and 'rebuked them for their lack of faith and their stubborn refusal to believe (apistia) those who had seen him after he had risen' (Mark 16:14). Even the disciples — eyewitnesses of Jesus' ministry — had to consciously choose to believe the resurrection testimony. Paul uses apisteo to address a crucial pastoral question: what happens to God's faithfulness if His covenant people are unfaithful? His answer is absolute: 'What if some were unfaithful? Will their unfaithfulness nullify God's faithfulness? Not at all! Let God be true, and every human being a liar' (Romans 3:3–4). Second Timothy 2:13 adds the final word: 'If we are faithless, he remains faithful, for he cannot deny himself.' God's trustworthiness is not contingent on human belief. Apisteo is humanity's failure; God's pistis (faithfulness) is immutable.