Immediately after the rich young man departs, the next words of Jesus are to his disciples. Matthew reformulates it as an “AMEN” saying. The word “Amen” occurs thirty-two times in Matthew. Beginning some of his pronouncements with “Amen” was a unique aspect of Jesus’ own authoritative speech. Amen is not a Greek word, but a transliteration of the Hebrew word “Amen” which is a responsive affirmation to something said previously. In this context, it is used to make the pronouncement of Jesus solemn. The pronouncement is about the impossibility of a rich person entering the kingdom of God. Jesus clearly reached for the most extreme illustration of impossibility, and the disciples got the point.
In response to Peter’s question, which must be seen as a continuation of the preceding dialogue (for taken by itself, Peter’s question seems purely selfish) Jesus affirms the eschatological reward for those who have not depended on their own goodness/talents/abilities/righteousness, but acknowledge their dependence on God’s free grace.
The point is not so much that God will prevent the rich from entering the kingdom, but that their riches will be an obstacle in their path.