The disciples ask Jesus again and again who will be first in the Kingdom. And it is no wonder that this question occupies them so much: they still could not fully break away from the ideas about the Messianic Kingdom that were widespread in those days. It seemed to them to be something not earthly by nature, yet quite earthly in everything that concerned relations between the King and His subjects.
Meanwhile, Jesus immediately points out their mistake, explaining that His Kingdom is completely unlike the kingdoms of this world: in His Kingdom, to be "above" means to serve those who are "below," and the King in this Kingdom voluntarily serves each of His subjects (vv. 25-28). And yet, valuing their sincere intention, Jesus does not discourage His disciples, does not force them to be silent or to give up. He does, however, tell them directly that they have no idea what they are asking for, because to sit at Jesus' right and left hand in His Kingdom means, first of all, to share with Him His way of the cross (vv. 22-23).
Still, the disciples apparently did not fully understand their Teacher. They may have thought they would have to share persecutions with Jesus, but they plainly did not expect Him to die the death He had to die. And they still imagined the Kingdom itself as something external, like what many people then expected, hoping that when the Messiah came He would drive the Romans from the sacred land and restore an independent Jewish state, where He would rule as God's Servant.
Jesus, seeing and understanding all this, nevertheless promises the apostles that they will share His cup (v. 23), and there is a logic to this. Indeed, no human being can ever imagine in advance the whole earthly path he will have to walk. If any one of us could see at the very beginning of his spiritual path everything that awaits him, hardly anyone would decide to set out. But such a picture would be inadequate from the start: seeing the difficulties ahead of us, we would not be able to imagine our own capacities, which at the beginning of the path we do not yet have. For as we walk the path, we ourselves change, so that when we enter a new stage and face new difficulties, we meet them no longer as we were in the beginning. But the goal of the path must be known from the very beginning; otherwise it is easy to lose the road - and perhaps it is not so terrible if that goal appears to us from a distance not as it will turn out to be when we come closer to it.
So it was with the apostles: they saw the goal of their path, they sought the Kingdom, and Jesus supports them, even though their ideas about the Kingdom are still rather vague and in some ways mistaken. Nothing can be done about this: there are truths that become understandable only when a certain part of the path has already been traveled. The disciples had the main thing: thirst for the Kingdom and trust in the Teacher. Everything else could be learned along the way.
