The father thought the crucial question was whether Jesus could heal the boy. Jesus explained that it was really whether the father believed that Jesus could heal him. This pinpointed the father"s understanding of who Jesus was. The issue was not how strongly the father believed Jesus would heal his son. This is an important distinction. Modern "faith healers" usually stress the amount of trust that the person coming for help has rather than the object of that trust. Later Jesus revealed that the disciples" failure to heal the boy resulted from lack of trust in Him too ( Mark 9:29). [source][source][source]
"One who has faith will set no limits to the power of God." [1][source]
"But the faith that has such mighty results will submit to the will of God in making its petitions. Faith-prompted prayer asks in harmony with the will of God." [2][source]
The father voiced his confidence in Jesus, imperfect as it was, and asked Jesus to strengthen his faith. [source][source][source]
"He declares that he believes and yet acknowledges himself to have unbelief. These two statements may appear to contradict each other but there is none of us that does not experience both of them in himself." [3][source]
He was an unbelieving believer, namely, a believer whose faith was weak. [source][source][source]