And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son.
Note 22 at Joh 14:13: If the previous promise didn't convince the disciples that the Father would accomplish the same miracles through them as He had through Jesus, then this one should have. Jesus reiterated the truth of Joh 14:12 in different words, thereby underscoring the certainty of this promise. Then in Joh 14:14, He repeated this same promise again.
It was very uncharacteristic of Jesus to repeat Himself three times in immediate succession. This must, no doubt, be interpreted as Jesus assuring His disciples that this seemingly unbelievable promise was, in fact, true. As explained in Heb 6:17-18, God did the same thing with Abraham. Anyone who would doubt the statement of Joh 14:12 because of it being such a departure from the norm could not, in a pure heart, doubt the meaning of Jesus' three statements when taken as a whole.