It is sad that Jesus finds it necessary to exhort the followers closest to him to believe his words, and therefore to believe that he is himself the revelation of the Father. Sad, indeed; but not strange. Is not our own unbelief proof enough of the commonness of unbelief? Even after we have been assured of God’s love for us again and again, of his sovereign pleasure to bless his people with what he judges good for them, do we not retreat to practical skepticism when difficult circumstances seem to call in question his goodness or his power?
Jesus’ first disciples in John 14 are experiencing difficulties of several kinds. They are perhaps intellectually slow to believe the daring claim on Jesus’ lips, made repeatedly, that he is in the Father and the Father in him. Worse, they are bound up emotionally as well as intellectually as they wrestle with talk about death, betrayal, Jesus’ departure, their inability to follow him at present, and the like. What they need more than anything else is to believe Jesus, to believe that what he is saying is true. If only they believe, then the uncertainties surrounding these other large matters will be swallowed up by confidence that Jesus is none other than the revelation of the Father. There is no belief more basic to spiritual triumph than that.
D.A. Carson, The Farewell Discourse and Final Prayer of Jesus