“Simon Peter said unto him, Lord, whither goest thou? Jesus answered him, Whither I go, thou canst not follow me now; but thou shalt follow me afterwards.”
Simon Peter asked him, 'Lord, where are you going?' Jesus answered, 'Where I am going, you cannot follow now; but you will follow afterward' — Peter senses the impending departure but cannot yet grasp its meaning or its finality; he does not understand that Jesus speaks of death and resurrection, of ascension beyond the reach of mortal following. The "now" and "afterward" establish the two phases of discipleship: now, while Jesus walks visibly with them; afterward, when they will follow him in death and resurrection, in the way of the cross. The postponement ("cannot follow now") is not punishment but merciful: the disciples must be strengthened and transformed before they can walk the path of ultimate sacrifice.
Church Fathers on John 13:36
Great is love, and stronger than fire; nothing can stop its course. Peter the most ardent of all, as soon as he hears our Lord say, Where I go you cannot follow Me now, asks, Lord, where do you go?
The disciple asks this, as if he were ready to follow. But our Lord saw his heart; Jesus answered him, Where I go, you cannot follow Me now; He checks his forwardness, but does not destroy his hope; nay, confirms it; But you shall follow Me afterwards. Why do you hasten, Peter? The Rock has not yet established you with His spirit. Be not lifted up with presumptions, you can not now; be not cast down with despair, you shall follow Me afterwards.
either as affirming, you shall lay down your life for My sake, but now through fear of the death of the body, you shall incur spiritual death: or as mocking; as if He said,