Mark 9:17-24. The disciples are unable to cast out a demon from this guy’s son, because this kind only comes out by prayer and fasting. One thing to note is that the dad first calls Jesus “teacher” and complains about the disciples. After Jesus makes a comment about this generation, the dad changes his tune and calls Jesus “Lord” and says “help my unbelief!” Remember that Judas calls Jesus “teacher”, but all the other disciples call him “Lord”.
Is Jesus your Lord?