…فَقُل لَّن تَخْرُجُوا مَعِيَ أَبَدًا وَلَن تُقَاتِلُوا مَعِيَ عَدُوًّا ۖ…  (Say, “You will not go out with me, ever, and you will never fight an enemy with me.): Allah (swt) states that it is not permissible for them to join the Prophet (saws) or engage in fighting against the enemies who attack al-Medina, because they are hypocrites, and their presence is a source of troubles. Their punishment is not limited to having their names removed from the records of those who accompany the Prophet (saws). There is another consequence, that is they are prohibited from being with the Messenger of Allah (saws) in defending the city of al-Medina.

…إِنَّكُمْ رَضِيتُم بِالْقُعُودِ أَوَّلَ مَرَّةٍ…  (you were satisfied with sitting [at home] the first time,): This is the reason for expelling them.

…فَاقْعُدُوا مَعَ الْخَالِفِينَ  (so sit [now] with those who stay behind.):  Those who stayed behind are either those who had lagged behind in joining the Messenger of Allah (saws), or had opposed the command of the Messenger (saws) to go out, or those who had become corrupted. Another Arabic word with a similar root-word, the Khaluf of the mouth of a fasting person, is the change in the smell in the mouth of a fasting person. The alteration of the smell signifies the spoilage and corruption of the substance. Therefore, the Arabic word of (stay behind) indicates that they had become corrupt and have opposed the orders of the Messenger of Allah (saws).