If the disciples did not understand it, what is this gospel of the kingdom that they were preaching? Obviously it was not the forgiveness of sins through the cross.
The whole ministry of Jesus encapsulates numerous truths and must be taken in the whole IMV....He was not only the Son of Man, but also the Son of God, the Kings of kings, Lord of lords and the SON of David at the same time.....the problem comes when people like some on this site decided to pit the words of Jesus against the words of Paul......this leads to numerous errors, conflated gospels and false gospels.......Jesus was clear....he had many things to say, but the disciples were not yet ready to hear it or bear up under it....the words that PAUL speaks are the words of JESUS and those words flesh out the truths that JESUS began in his ministry.......this is what MANY deny, miss or fail to see.......John the Baptist prepared the material, JESUS organizes the material and lays the ground work and principles while PAUL finishes it off with the expanded, fleshed out line of truth......of course Peter, John, Jude, James focuses in on particular applications of the word...James to the Jews scattered, Jude to all concerning the common salvation open to all, Peter to both and John to both.......
The entire N.T. is taken as a whole to ALL.......It just so happens that JESUS being a JEW and of the line of David and tribe of Judah begins his ministry in Israel.........and then uses PAUL to expand that ministry from within Israel out to the Gentiles from Rome to Turkey......but note.....PAUL almost without exception started in the synagogues and then worked outwardly....BOTH JEWS and GENTILES included even though he was primarily sent unto the Gentiles....
The base message......the Kingdom of God is in you and this is how you enter the kingdom and these are the characteristics of the subjects of the kingdom and this is the reward for faithful service to the KING.....which happens to be both the KING of the JEWS and the KING of the world