What I do. We are taught by these words, that we ought simply to obey Christ, even though we should not perceive the reason why he wishes this or that thing to be done. In a well-regulated house, one person, the head of the family, has the sole fight to say what ought to be done; and the servants are bound to employ their hands and feet in his service. That man, therefore, is too haughty, who refuses to obey the command of God, because he does not know the reason of it. But this admonition has a still more extensive meaning, and that is, that we should not take it ill to be ignorant of those things which God wishes to be hidden from us for a time; for this kind of ignorance is more learned than any other kind of knowledge, when we permit God to be wise above us.

Thou knowest not now - Though he saw the action of Jesus, yet he did not fully understand the design of it. It was a symbolical action, inculcating a lesson of humility, and intended to teach it to them in such a manner that it would be impossible for them ever to forget it. Had he simply commanded them to be humble, it would have been far less forcible and impressive than when they saw him actually performing the office of a servant.
Shalt know hereafter - Jesus at that time partially explained it John 13:14-15; but he was teaching them by this expressive act a lesson which they would continue to learn all their lives. Every day they would see more and more the necessity of humility and of kindness to each other, and would see that they were the servants of Christ and of the church, and ought not to aspire to honors and offices, but to be willing to perform the humblest service to benefit the world. And we may remark here that God often does things which we do not fully understand now, but which we may hereafter. He often afflicts us; he disappoints us; he frustrates our plans. Why it is we do not know now, but we yet shall learn that it was for our good, and designed to teach us some important lesson of humility and piety. So he will, in heaven, scatter all doubts, remove all difficulties, and show us the reason of the whole of his mysterious dealings in his leading us in the way to our future rest. We ought also, in view of this, to submit ourselves to him; to hush every murmur, and to believe that he does all things well. It is one evidence of piety when we are willing to receive affliction at the hand of God, the reason of which we cannot see, content with the belief that we may see it hereafter; or, even if we never do, still having so much confidence in God as to believe that what He does is right.

What I do thou knowest not now, etc. - As if our Lord had said, Permit me to do it now, and I will shortly explain to you the nature of this action, and my motives for doing it.
Thou shalt know hereafter - Μετα ταυτα, after this business is finished. And so we find he explained the whole to them, as soon as he had finished the washing: see John 13:12-17. I cannot think that this refers to any particular instruction received on this head after the day of pentecost, as some have conjectured.

Jesus answered and said unto him,.... Christ replies,
what I do, thou knowest not now: Peter knew that he was about to wash his feet, and the rest of his disciples, but he did not know the meaning and mystery of it, what Christ designed by it, and what instruction they were to receive from it,
but thou shalt know hereafter; as he did, when he had performed this service, and explained it to him. This may teach us, under dark providences, the meaning of which is not yet known by us, to wait the Lord's own time, to make things clear and plain to us, and in the mean time patiently submit to the divine will.

Jesus answered and said . . . What I do thou knowest not now--that is, Such condescension does need explanation; it is fitted to astonish.
but thou shall know hereafter--afterwards, meaning presently; though viewed as a general maxim, applicable to all dark sayings in God's Word, and dark doings in God's providence, these words are full of consolation.

What I do thou knowest not now; but thou shalt know hereafter - We do not now know perfectly any of his works, either of creation, providence, or grace. It is enough that we can love and obey now, and that we shall know hereafter.