“This kind can only come out through prayer”
When God says…“that which you ask the Father in my name, he will give it to you,” it is implied that that which is asked should be necessary for salvation. thus Rabanus in his commentary on Matthew says: No matter how often you ask the Father, he will not hear you if what you ask for is something that hinders your salvation. Or again, he may not seem to hear you because the answer to your petition is delayed to some future time, either so that you might develop greater yearning, or so that the joy which he withholds might be the more completely apprehended.
It is necessary to ask with piety. This phrase implies all the attitudes which are necessary on the part of the one that prays—insofar as he prays at all. Namely, he should pray with faith, with humility, and with fervor. First, he must pray with faith and confidence, for, as Blessed Jacobus says: Ask with faith and without hesitance. Second, he must pray with humility, for it says in Psalm 101: “He has regard to the prayer of the humble, and he has not despised their petitions.” Third, he should pray fervently, for as Saint Augustine says: Greater worth follows prayer when it is preceded with a fervent disposition.