"And there were certain Greeks (Gentile converts to Judaism) out of those coming up to worship in the feast. These then came near to Philip, who is from Bethsaida of Galilee, and were asking him, saying, 'Sir, we wish to see Jesus.' Philip cometh and telleth Andrew, and again Andrew and Philip tell Jesus. And Jesus responded to them, saying, 'The hour hath come that the Son of Man may be glorified. Verily, verily, I say to you, if the grain of the wheat, having fallen to the earth, may not die, itself remaineth alone; and if it may die, it doth bear much fruit. He who is loving his life shall lose it, and he who is hating his life in this world, shall keep it. If any one may minister to Me, let him follow Me, and where I am, there also My servant shall be; and if any one may minister to Me, the Father will honor him. Now hath My soul been troubled, and what shall I say? Father, save Me from this hour? No, it is because of this I came to this hour. Father, glorify Thy name.' There came, therefore, a voice out of the heaven saying, 'I both glorified, and again I will glorify it.' The multitude, therefore, having stood and heard, were saying that there hath been thunder. Others said, 'A messenger (an angel) hath spoken to Him.' Jesus answered and said, 'Not because of Me hath this voice come, but because of you. Now is a judgment of this world, now shall the ruler of this world be cast forth. And I, if I may be lifted up from the earth, will draw all men unto Myself.' And this He said signifying by what death He was about to die" (Jn. 12:20-33 - based upon Young's Literal Translation).Amazingly, even after the Crucifixion had taken place, the disciples did not understand Jesus had to die. In Luke 24:13-48, we are told of two of them who had given up hope that Jesus was their Messiah, and being totally discouraged, they decided to return home. When Jesus appeared to them, it was not until He broke the bread that they recognized Him. Perhaps they were so distraught that their minds would not let them see Him standing before them. Regardless of the reason, when Jesus appeared to them again following their return to Jerusalem to tell the other disciples, they were all afraid, "believing that they had seen a spirit" (v. 37).
You would think the Lord suddenly appearing in a locked room (Jn. 20:19) would bring them joy, but it only brought them fear. It was not until He "opened their understanding" that they were able to believe and rejoice (v. 45). Today, it still takes Jesus "opening our understanding" for us to comprehend who He is, why He suffered and died for our sin, and to believe in Him as our resurrected Lord and Savior. Until He does that for you, you will not be able to believe either.