Now when He got into a boat, His disciples followed Him. And suddenly a great tempest arose on the sea, so that the boat was covered with the waves. But He was asleep. Then His disciples came to Him and awoke Him, saying, “Lord, save us! We are perishing!”

But He said to them, “Why are you fearful, O you of little faith?”  Then He arose and rebuked the winds and the sea, and there was great calm.  Matthew 8:23-26.

Little faith, oligopistos; Strong’s # 3640. A combination of oligos, “small,” and pistis, “faith,” used to describe a faith that lacks confidence or trusts too little. Jesus used the word in various situations as a tender rebuke or corrective chiding (Matt. 6:30; 8:26; 14:31; 16:8; Luke12:28).

A kinder explanation would be to say their faith was “underdeveloped faith” as opposed to outright unbelief or distrust which is apistis.