Then Jesus said unto them, Yet a little while is the light with you. Walk while ye have the light, lest darkness come upon you: for he that walketh in darkness knoweth not whither he goeth. While ye have light, believe in the light, that ye may be the children of light. These things spake Jesus, and departed, and did hide himself from them.
John 12:35-36

Barnes' Notes on the Bible
Yet a little while is the light with you - Jesus did not reply directly to their question. He saw that they were offended by the mention of his death, and he endeavored to arrive at the same thing indirectly. He tells them, therefore, that the light would be with them a little while, and that they ought to improve the opportunity while they had it to listen to his instructions, to require with candor, and thus to forsake their false notions respecting the Messiah.
The light - John 1:4. It is probable that they understood this as denoting the Messiah. See John 8:12; "I am the light of the world;" John 9:4.
Walk ... - John 11:9. Whatever you have to do, do it while you enjoy this light. Make good use of your privileges before they are removed. That is, while the Messiah is with you, avail yourselves of his instructions and learn the way to life.
Lest darkness - Lest God should take away all your mercies, remove all light and instruction from you, and leave you to ignorance, blindness, and woe. This was true that darkness and calamity were to come upon the Jewish people when the Messiah was removed; and it is also true that God leaves a sinner to darkness and misery when he has long rejected the gospel.

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Clarke's Commentary on the BibleYet a little while is the light with you - In answer to their objection, our Lord compares himself to a light, which was about to disappear for a short time, and afterwards to shine forth with more abundant lustre; but not to their comfort, if they continued to reject its present beamings. He exhorts them to follow this light while it was among them. The Christ shall abide for ever, it is true; but he will not always be visible. When he shall depart from you, ye shall be left in the thickest darkness; in impenitence and hardness of heart. Then shall ye wish to see one of the days of the Son of man, and shall not see it, Luke 17:22. Then shall ye seek me, but shall not find me, John 7:34. For the kingdom of God shall be taken from you, and given to the Gentiles, Matthew 21:43. If ye believe not in me now, ye shall then wish ye had done it, when wishing shall be for ever fruitless.