What does Mark 10:39 mean?
Luke's account of Jesus' third prophecy of His death and resurrection mentions that God hides the meaning of Jesus' words from the disciples (Luke 18:34). Although the disciples remember Jesus' words later (Luke 24:6–8), and they have some inkling of the danger of the situation (John 11:16), they do not fully understand what Jesus is saying. If they had, there is no telling how the next few days would have been altered; it's likely the triumphal entry, Jesus' teaching in Jerusalem, and the Last Supper would have happened very differently. The Twelve do not permanently receive the Holy Spirit and His guidance until after Jesus' ascension (Acts 2:1–4; see John 16:7). God, it seems, may choose to hide His plan from even those of us with the Holy Spirit, when the knowledge would incite us to ruin that plan with our presumption or fear.James and John do suffer, although not exactly like Jesus. James is the first of the Twelve to be martyred when King Herod kills him with a sword (Acts 12:1–2). The Bible does not record John's fate. Tradition says that at one point he is placed in a cauldron of burning oil. He not only survives, he continues preaching while still in the pot. He is then exiled to the island of Patmos where he writes the book of Revelation. Eventually, he is released and dies of old age.
There is nothing humans can do to add to or fulfill Jesus' work on the cross. We are incapable of doing anything to earn salvation (Ephesians 2:8–9). Paul does speak of our necessary sacrifice, however: "Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ's afflictions for the sake of his body, that is, the church" (Colossians 1:24) The work Christians do is not to fulfill the gospel, but to "make the word of God fully known" (Colossians 1:25). Whenever we drink the cup or accept the baptism of suffering and sacrificial leadership, it is not to "do" the work of salvation from sin. Rather, it is to spread the news that Jesus has already completed it.