Albert Barnes on Matthew 21:22

And all things, etc. He adds an encouragement for them to pray, assuring them that they should have all things which they asked. This promise was evidently a special one, given to them in regard to working miracles. To them it was true. But it is manifest that we have no right to apply this promise to ourselves. It was designed specially for the apostles; nor have we a right to turn it from its original meaning.

(m) "in prayer" Mt 7:7, Mk 11:24, Jas 5:16, 1Jn 3:22, 5:14