What was the mysterious cup that Christ fervently prayed would "pass from me," meaning he would not have to experience the fullness of what it symbolized (see verse 42)?
Jesus requested of the Father "this cup pass from me" when he, in the Garden of Gethsemane, prayed just before being betrayed by Judas Iscariot and arrested. Was He fearful regarding the physical abuse he would receive before being condemned to die? Was he concerned that his suffering would tempt him to SIN? Was he trying to avoid the painful death on the cross the he was prophesied to experience? Just before he prayed what caused him to state, "My soul is deeply grieved, even to death." (Matthew 26:38)?
Was Jesus afraid of dying?
Many times, it is difficult for humans to face their impending death. Jesus, however, all during his ministry (and possibly many years before it) not only KNEW he would have to die as the Lamb of God but also had perfect faith that the Father would raise him from the dead.
'Behold, we are going up to Jerusalem, and the Son of Man will be betrayed to the chief priests and to the scribes; and they will condemn Him to death and deliver Him to the Gentiles . . . And the third day He will rise again.' (Mark 10:32 - 34)
" . . . The Son of Man must suffer many things . . . and be killed, and be raised the third day."" (Luke 9:22, see also Matthew 16:21, 17:22 - 23)
Jesus was not afraid of suffering and dying, of itself, as it was for these purposes that he divested himself of his eternal existence as part of the Godhead to assume the form of a human (see Hebrews 2:9, 10:10, 12, 14)!
The wine of his wrath
If Jesus did not agonize over his impending suffering and death, then what did he desire to avoid? The New Testament DOES discuss a terrible cup that, in the near future, sinning humans will be forced to drink.
9. And a third angel followed them, saying with a loud voice, "If anyone worships the beast and his image, and receives the mark in his forehead or in his hand, 10. He shall also drink of the wine of the wrath of God, which is mixed undiluted in the cup of His wrath . . ." (Revelation 14:9 - 10, HBFV)
The cup Jesus dreaded to drink from was his Father's wrath against sin. Nelson's Bible Commentary on Matthew 26:36 - 39 makes an interesting comment in this regard.
". . . Jesus questions the will of the Father as to the necessity of drinking the cup. While this may refer to death ("he tasted death") it is more likely that the cup represents the wrath of man's sin-bearer. In the awful anguish of that moment, the sin of the world was poured on Christ and He became "sin for us" (2Corinthians 5:21). Thus, the Righteous One dies a substitution death for guilty mankind."
By taking upon himself the sins of the entire world Christ not only made forgiveness possible but also made available protection from God's wrath.
8. But God commends His own love to us because, when we were still sinners, Christ died for us. 9. Much more, therefore, having been justified now by His blood, we shall be saved from wrath through Him. (Romans 5:8 - 9)
The apostle Paul further confirms this fact when he writes to the church in Thessalonica.
" . . . and to wait for His Son from heaven, whom He raised from the dead, even Jesus who delivers us from the wrath to come." (1Thessalonians 1:10)
"For God did not appoint us to wrath, but to obtain salvation through our Lord Jesus Christ, . . . " (5:9)
The sin of man
Near the end of Christ's six-hour ordeal on the cross, he cried out the following that has a direct bearing on the "cup" he ultimately drank.
45. Now from the sixth hour until the ninth hour, darkness was over all the land. 46. And about the ninth hour, Jesus cried out with a loud voice, saying, "Eli, Eli, lama sabachthani?" That is, "My God, My God, why have You forsaken Me?" (Matthew 27:45 - 46, HBFV)
What caused the Father to FORSAKE Jesus at this critical moment when He was with him all during his physical life? It seems highly likely that it was at this moment that God placed the sins of ALL humans upon Christ.
After Jesus literally BECAME sin (2Corinthians 5:21, 1John 3:5, etc.) the eternal contact he enjoyed with the Father was severed (it is sin that separates anyone from God, see Isaiah 59:2) and he experienced God's wrath for sin. When Christ requested, "let this cup pass from me," he was asking that, if it were possible, that he would not have to experience separation from God and have to endure his wrath against sin.