John 12:23-24
“The hour has come, that the Son of Man should be glorified. Verily, verily, I say unto you, Except a grain of wheat fall into the ground and die, it abides alone.”
What is, “The hour has come”? He had said, “Go not into the way of the Gentiles,” (thus cutting away all excuse of ignorance from the Jews,) and had restrained the disciples. When therefore the Jews continued disobedient, and the others desired to come to Him, “Now,” says He, “it is time to proceed to My Passion, since all things are fulfilled. For if we were to continue to wait for those who are disobedient and not admit these who even desire to come, this would be unbefitting our tender care.” Since then He was about to allow the disciples to go to the Gentiles after the Crucifixion, and beheld them springing on before, He said, “It is time to proceed to the Cross.” For He would not allow them to go sooner, that it might be for a testimony unto them. Until that by their deeds the Jews rejected Him, until they crucified Him, He said not, “Go and make disciples of all nations”, but, “Go not into the way of the Gentiles”, and, “I am not sent but unto the lost sheep of the house of Israel”, and, “It is not meet to take the children's bread and give it unto dogs.” But when they hated Him, and so hated as to kill Him, it was superfluous to persevere while they repulsed Him. For they refused Him, saying, “We have no king but Cæsar.” So that at length He left them, when they had left Him. Therefore He says, “How often would I have gathered your children together, and you would not?”
What is, “Except a grain of grain fall into the ground and die”? He speaks of the Cross, for that they might not be confounded at seeing, that just when Greeks also came to Him, then He was slain, He says to them, “This very thing specially causes them to come, and shall increase the preaching of Me.” Then since He could not so well persuade them by words, He goes about to prove this from actual experience, telling them that this is the case with grain; it bears the more fruit when it has died. “Now,” says He, “if this be the case with seeds, much more with Me.” But the disciples understood not what was spoken. Wherefore the Evangelist continually puts this, as making excuse for their flight afterwards. This same argument Paul also has raised when speaking of the Resurrection.
Source: Homilies on the Gospel of John (New Advent)