"Then Jesus said to them again, 'I am going away, and you will seek Me, and will die in your sin. Where I go you cannot come.' So the Jews said, 'Will He kill Himself, because He says, "Where I go you cannot come"?' And He said to them, 'You are from beneath; I am from above. You are of this world; I am not of this world. Therefore I said to you that you will die in your sins; for if you do not believe that I am He, you will die in your sins.' Then they said to Him, 'Who are You?' And Jesus said to them, 'Just what I have been saying to you from the beginning. I have many things to say and to judge concerning you, but He who sent Me is true; and I speak to the world those things which I heard from Him.' They did not understand that He spoke to them of the Father. Then Jesus said to them, 'When you lift up the Son of Man, then you will know that I am He, and that I do nothing of Myself; but as My Father taught Me, I speak these things. And He who sent Me is with Me. The Father has not left Me alone, for I always do those things that please Him.' As He spoke these words, many believed in Him" (John 8:21-30).
Jesus spoke on this theme several times (cf. John 7:34; 13:33) - "I am going away, and you will seek Me, and will die in your sin." It wouldn't be long before Jesus would be crucified, buried, and resurrected. Then He would "go away" by ascending to the Father and no one could follow Him there - at least not yet (cf. 14:2,3). The Jews would not seek Him specifically, but they would continue to seek their Messiah. However, since Jesus was the Messiah, their seeking for Him elsewhere would be in vain and they would end up dying lost in sin. This is the reason why they couldn't go to heaven where Jesus would be going. No accountable person today can go to heaven without knowing where Jesus came from and where He went.
Certainly they understood Jesus to be talking about the afterlife. The common Jewish belief was that those who committed self-murder (i.e., suicide) went into the deepest spiritual depths of destruction. Thus, they are trying to reverse the meaning of Jesus' statement with mockery. Their meaning is this: "Well Jesus, if we can't go where you are going that must mean that you are going to take your own life and end up in a spiritual place of torment where we will never go!"
Jesus responded in John 8:23 - "You are of this world; I am not of this world." Not only were they from beneath so far as their origin is concerned, but their mindset was also of this world; that is, seemingly unable to discern spiritual things.
"If you do not believe that I am He, you will die in your sins" (John 8:24). Because of their mindset, they refused to believe in Jesus as the Christ, and the consequence of this would be eternal condemnation!
Jesus' bold call for them to believe in Him led them to make a counter demand, namely that He confess His identity to them. Of course, all along Jesus had been doing this! He had plainly told them that He was the bread of life, the water of life, and the light of the world. There was more He could say, but now was not the time. These Pharisees were insincere in their question anyway. They didn't really want to know who Jesus was, but rather they wanted Him to affirm something they could use against Him.
Jesus had primarily been revealing Himself through His teachings. However, in John 8:26 He states that He has "many things to say and to judge" concerning them, His enemies. Although these words would not be pleasant, they were given to Him by the Father, and they were true regardless of whether or not they were accepted. Friends, we, like Jesus, must speak out boldly in God's name. We should not purposely intend to be offensive, but neither can we compromise the truths that God has given us in His Scriptures.
Although the Pharisees lacked understanding, Jesus continued anyway - "When you lift up the Son of Man, then you will know that I am He, and that I do nothing of Myself; but as My Father taught Me, I speak these things" (cf. 3:14; 12:32). Here Jesus alludes to His crucifixion and the fact that after He was slain there would be those who finally would understand His true identity as the Messiah. They would know that He was a true servant of the Father and that He only did and taught those things that the Father had instructed Him to. This aspect of the prophecy was fulfilled on the Day of Pentecost.
"And He who sent Me is with Me. The Father has not left Me alone, for I always do those things that please Him" (John 8:29). Though Jesus had left heaven, there was still a sense in which He and the Father were together (cf. 17:21). Jesus was always obedient to His Father, doing the things that pleased Him (cf. 4:34; II Cor. 5:9).
Jesus' words were convincing to those in the crowd who had good and honest hearts. They "believed in Him," but their faith was weak, as the following verses will show. When Jesus attempts to strengthen their faith through further teaching, they soon return to their state of doubt and questioning, and eventually to opposing Him and His teaching.