Augustine of Hippo — as excerpted in the Catena Aurea on John 14:12-14

“(Tract. lxxii. 2) Whatsoever ye shall ask. Then why do we often see believers asking, and not receiving? Perhaps it is that they ask amiss. When a man would make a bad use of what he asks for, God in His mercy does not grant him it. Still if God even in kindness often refuses the requests of believers, how are we to understand, Whatsoever ye shall ask in My name, I will do? Was this said to the Apostles only? No. He says above, He that believeth on Me, the works that I do shall he do also. And if we go to the lives of the Apostles themselves, we shall find that he who laboured more than they all, prayed that the messenger of Satan might depart from him, but was not granted his request. But attend: does not our Lord lay down a certain condition? In My name, which is Christ Jesus. Christ signifies King, Jesus, Saviour. Therefore whatever we ask for that would hinder our salvation, we do not ask in our Saviour’s name: and yet He is our Saviour, not only when He does what we ask, but also when He does not. When He sees us ask any thing to the disadvantage of our salvation, He shews Himself our Saviour by not doing it. The physician knows whether what the sick man asks for is to the advantage or disadvantage of his health; and does not allow what would be to his hurt, though the sick man himself desires it; but looks to his final cure. And some things we may even ask in His name, and He will not grant them us at the time, though He will some time. What we ask for is deferred, not denied. He adds, that the Father may be glorified in the Son. The Son does not do any thing without the Father, inasmuch as He does it in order that the Father may be glorified in Him.”