João 8:21

Evidently what follows continues Jesus’ teaching in the temple when He spoke the words that John recorded in the preceding verses. The Greek word palin ("again" or "once more") indicates a pause but not a significant break in the narrative (cf. John 8:12). The content of His teaching in this verse recalls John 7:33-34.

When Jesus said He was going away He was speaking of His death, resurrection, and ascension into heaven. The Jewish leaders would not seek Jesus personally, but they would continue to seek the Messiah. They would die in their sin (singular) of unbelief because they rejected Jesus. Jesus was going to His Father in heaven. These Jews could not come there because they had rejected Jesus.