Mark 9:29
He replied, this kind can come out only by prayer — the answer is a single word of diagnosis: prayer. The disciples had been given authority (Mark 6:7) and had exercised it successfully (6:13), but the kind of spirit in this boy — ancient, deeply embedded, destructive from childhood — required the prayerful dependence that the disciples apparently attempted without. The principle is not about exorcism technique but about the disciples' operating posture: they acted on their own authority rather than dependence on the Father through prayer.