that is a good question...this is referring to unbelief of a miracle. We can believe in the Triune God of the bible, and yet there are daily situations where we may doubt God's awesome power and might to conquer. The verse quoted was from a man who had a son with a demon in him and the man wanted Jesus to cast out the demon. Here is the full account: Mark 9:21-27. So He (jesus) asked the boy's father, “How long has this been happening to him?”
And the man said, “From childhood. And often he has thrown him both into the fire and into the water to destroy him. But if You can do anything, have compassion on us and help us.”
Jesus said to him, “If you can believe, all things are possible to him who believes.”
Immediately the father of the child cried out and said with tears, “Lord, I believe; help my unbelief!”
When Jesus saw that the people came running together, He rebuked the unclean spirit, saying to it, “Deaf and dumb spirit, I command you, come out of him and enter him no more!” Then the spirit cried out, convulsed him greatly, and came out of him. And he became as one dead, so that many said, “He is dead.” But Jesus took him by the hand and lifted him up, and he arose.
