Summary: Jesus confronts the unbelieving crowd and warns that those who reject Him have a certain expectation of judgment ending in death, but those who believe in Him will live eternally with Him.
The overall purpose of John is to encourage us to believe that Jesus is the Son of God and the Christ; the promise is that those who believe this will live. The immediate context of chapter twelve is Jesus preparing His disciples for His death and resurrection. The Greeks have come signaling the final hour, and He tells His disciples that He must die to bear much fruit and that they must follow Him. Now He says,
Now is my soul troubled; and what shall I say? Father, save me from this hour: but for this cause came I unto this hour.
He’s troubled over His pending death, but He knows this is His whole purpose in coming. If it’s possible He wants this cup to pass, but He wants even more for the will of the Father to be done (Mt. 26:39):
28Father, glorify thy name.
It seems like a strange thing to say here, but consider from other verses how the Father is glorified:
Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples (Jn. 15:8).
These words spake Jesus, and lifted up his eyes to heaven, and said, Father, the hour is come; glorify thy Son, that thy Son also may glorify thee: 2As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him. 3And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent. 4I have glorified thee on the earth: I have finished the work which thou gavest me to do. 5And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was (Jn. 17:1-5).
Jesus hasn’t yet died, but He says He finished the work He was sent to do. It’s not that there’s nothing left to be done, but His primary goal (as the image of God) is to reveal the Father to the children of God. This glorifies the Father and Jesus accomplished it.
Look at one more:
That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; 11And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father (Phil. 2:10-11).
The death of Christ merely precedes the fruit which follows; Christ’s resurrection (and ours) glorifies the Father.
Then came there a voice from heaven, saying, I have both glorified it, and will glorify it again.
I can only find two other places during Christ’s earthly ministry where the Father speaks from heaven: Christ’s baptism (the start of His public ministry) and at His transfiguration (when He revealed Himself in glorified form). The significance of those two occasions is evident, but this time seems a little insignificant. Why would the Father choose to speak from heaven now?
When was He glorified, and when will He be again? The answer is revealed in the following verses:
29The people therefore, that stood by, and heard it, said that it thundered: others said, An angel spake to him.
Taking verse 37 into account, we know that these people didn’t believe Jesus. It’s interesting that some of the people heard something they thought was just an occurrence of nature and others thought it was something spiritual, but neither one recognized the voice of the Father.