What does Matthew 20:23 mean?
Using their mother to make the request on their behalf, James and John have asked Jesus for something tremendous (Matthew 20:20–22). They want to be named as His seconds in command and power when His kingdom is established. He has asked, symbolically, if they are able to endure what He is about to experience. They responded with a yes and now Jesus says that they will "drink His cup." In other words, Jesus is saying that James and John will experience suffering as He will. This does not mean that they will experience all the suffering of Jesus, to the exact degree and severity of His experience on the cross. However, He is suggesting they will suffer for Him, on His behalf (John 15:20; 16:1–3).And suffer they would. As it turns out, these two men represent the first and last of the loyal disciples to die. James was the first of the disciples to be killed for Jesus' sake (Acts 12:2). John the Apostle was persecuted and eventually exiled to the island of Patmos "on account of the word of God and the testimony of Jesus" (Revelation 1:9). So far as we know, one of these brothers will die relatively soon, the other will outlive all the others.
Still, Jesus cannot grant their request to sit on His left and on His right in the kingdom. Only God the Father can grant that request. This shows that Jesus understands Himself to be under the authority of His Father in this, though they are one (John 14:8–11). He adds that His Father has prepared those seats for someone, and that is who will occupy them. If Jesus knows who those two people are, He chooses not to reveal it here.