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.

Knowest - shalt know (οἷδας - γνώσῃ)

The A.V. ignores the distinction between the two words. "Thou knowest not" (οὐκ οἶδας), of absolute and complete knowledge. Thou shalt learn or perceive (γνώσῃ), of knowledge gained by experience. See on John 2:24.

Hereafter (μετὰ ταῦτα)

Literally, after these things.

Jesus answered and said unto him, What I do thou knowest not now; but thou shalt know hereafter.

13:7 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.

7. 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.

13:1-17 Our Lord Jesus has a people in the world that are his own; he has purchased them, and paid dear for them, and he has set them apart for himself; they devote themselves to him as a peculiar people. Those whom Christ loves, he loves to the end. Nothing can separate a true believer from the love of Christ. We know not when our hour will come, therefore what we have to do in constant preparation for it, ought never to be undone. What way of access the devil has to men's hearts we cannot tell. But some sins are so exceedingly sinful, and there is so little temptation to them from the world and the flesh, that it is plain they are directly from Satan. Jesus washed his disciples' feet, that he might teach us to think nothing below us, wherein we may promote God's glory, and the good of our brethren. We must address ourselves to duty, and must lay aside every thing that would hinder us in what we have to do. Christ washed his disciples' feet, that he might signify to them the value of spiritual washing, and the cleansing of the soul from the pollutions of sin. Our Lord Jesus does many things of which even his own disciples do not for the present know the meaning, but they shall know afterward. We see in the end what was the kindness from events which seemed most cross. And it is not humility, but unbelief, to put away the offers of the gospel, as if too rich to be made to us, or too good news to be true. All those, and those only, who are spiritually washed by Christ, have a part in Christ. All whom Christ owns and saves, he justifies and sanctifies. Peter more than submits; he begs to be washed by Christ. How earnest he is for the purifying grace of the Lord Jesus, and the full effect of it, even upon his hands and head! Those who truly desire to be sanctified, desire to be sanctified throughout, to have the whole man, with all its parts and powers, made pure. The true believer is thus washed when he receives Christ for his salvation. See then what ought to be the daily care of those who through grace are in a justified state, and that is, to wash their feet; to cleanse themselves from daily guilt, and to watch against everything defiling. This should make us the more cautious. From yesterday's pardon, we should be strengthened against this day's temptation. And when hypocrites are discovered, it should be no surprise or cause of stumbling to us. Observe the lesson Christ here taught. Duties are mutual; we must both accept help from our brethren, and afford help to our brethren. When we see our Master serving, we cannot but see how ill it becomes us to domineer. And the same love which led Christ to ransom and reconcile his disciples when enemies, still influences him.