Can you believe that some false teachers dispute what the Lord's Bitter Cup was, which He described on the night that He was betrayed unto death?
Christ told His disciples, and the mob that came to take Him away, "The cup which My Father hath given me, shall I not drink it?"
This was in response to Peter's attempt to use violence to block Jesus from being taken, falsely accused, found guilty and crucified. Jesus declared He would do His Father's Will, which was to drink that bitter cup of wrath and judgment to save His people.
The reason false teachers object to Christ's clear teaching, is that they reject substitutionary atonement. They refuse to believe God's Word, that Jesus was punished by God for the sins of His people that were laid upon Him at the cross.
Heretics don't like the idea that God's wrath fell on Jesus in our place. Therefore, they will not believe that the cup pictured wrath and judgment poured out by God upon His Dear Son.
Instead, these false teachers try to claim that the cup Christ refers to is somehow a joyous cup, perhaps "Elijah's Cup" of the Passover, which is left for Elijah, when he comes back to announce Messiah.
Often in Scripture, the metaphor of a cup is used to describe God's wrath poured out on sinners. Several times in the Revelation, and also in the prophets, judgment for sin from the hand of God is pictured as a cup of wrath.
In the immediate context, Christ had several times made explicit references to His imminent suffering and death. "With desire I have desired to eat this Passover before I suffer." "The Son of Man goeth, as it was determined, but woe to that man by whom He is betrayed!"