This passage looks back to Matthew 20:22, Jesus tells James and John that they are not able to drink the cup that he is about to drink. In that context, the cup refers to the coming crucifixion, and James and John both swear that they can “drink that cup.” Like Peter in the previous paragraph, they swore to go to their deaths alongside Jesus. Now and the time has come for them to keep watch, they fail. All three will fall asleep in the garden, and all three will flee when Jesus is arrested.
Jesus takes Peter, James, and John and asks them to sit with him to watch (26:38) while Jesus prays. Why these three disciples? The three disciples are usually called Jesus’s “inner circle” since out of the twelve, they are almost the only disciples mentioned by name and are the only disciples featured in some episodes. These three disciples accompanied Jesus during the transfiguration (Matthew 17). This is significant because they say the glory of God. They heard the voice from heaven and saw Moses and Elijah. After that revelation of who Jesus really was, they were the three disciples who ought to understand most clearly what was about to happen in the next few hours.
More importantly, all three boasted they would suffer alongside Jesus. In Matthew 20:20-28 the sons of Zebedee requested to sit on either side of Jesus when he comes into his kingdom. Jesus’s response anticipates Gethsemane. He tells them they are not able “to drink the cup that I am going to drink.” Both were indignant, and in Matthew 20:22, they claim they can drink “the same cup as Jesus.” Peter had a similar boastful moment when he claimed he would not fall away, only a few hours earlier (Matt 26:31-35). Looking ahead to the book of Acts, Luke only mentions Peter and John in Acts 2-5 and then informs the reader James was killed by Herod Agrippa (Acts 12:1).
The disciples are told to “keep watch” (γρηγορέω). The word has the sense of alertness, to “stay awake.” It was used for literal “guard duty” (1 Macc 12:27) but is used in Christian literature for spiritual alertness (1 Cor 16:13, for example). In Matthew 24:43, Jesus tells his disciples to “keep watch” because they do not know the time of his return (Parable of the thief in the night). The command is illustrated in the parable of the Ten Bridesmaids (Matt 25:1-13, which ends with those words).
Jesus says his soul is “overwhelmed with sorrow to the point of death” and asks them to stay and keep watch (Matthew 26:37-38). This is a rare word (περίλυπος). It is sorrow that covers one up, a sorrow that could cause death itself. It is sometimes translated as “afflicted beyond measure” The same phrase is used in Jonah 4:9; Jonah is so sorrowful that he would rather be dead. LXX Psalm 42:6 uses this word, “My soul is cast down within me.” It describes the reaction of the wise men in Babylon when they learn Nebuchadnezzar was going to execute them (LXX Dan 2:12). God asked Cain why he was so deeply grieved (LXX Genesis 4:6, most ET follow the Hebrew, he is angry).
Verse 38 adds that Jesus began to be “sorrowful and troubled.” Sorrowful is a common word in the NT for sorrow, but “troubled” (ἀδημονέω) is much more intense, sometimes translated as “tormented” or “disquieted” in ancient Greek (BrillDAG). Luke adds that Jesus sweats great drops of blood (Luke 22:44).
This agony in the Garden before his arrest, suffering, and crucifixion is a clear demonstration of the humanity of Jesus.