Those who reject the apostles will certainly be punished on the day of judgment (Matthew 7:22), and to a greater degree than Sodom and Gomorrah, because they rejected the gospel (Matthew 10:15).
Jesus already refers to the day of judgment in Matthew 7:22. By reminding them thereof again in Matthew 10:15, Jesus pronounces an eschatological judgment. Jesus teaches the apostles that rejecting the message of the gospel will never go unpunished. Jesus highlights this judgment to the disciples not to provide them with a sense of satisfaction, but rather to awaken their dedication to the cause, even in the midst of setbacks.1
It seems that even in the eternal judgment of God there are different levels of suffering. The reason that the homes and cities which reject the gospel would be judged more severely than Sodom and Gomorrah, is because where there is more light or greater revelation, there is also more responsibility. If somebody refuses to listen to this message which can save their lives, then they will inevitably have to hear and receive judgment.2
11 And whatever town or village you enter, find out who is worthy in it and stay there until you depart.