128. "Those who sit at home' (i.e. remain passive) does not refer either to those who had been ordered to fight but tried to look for excuses not to fight or to those who were individually obliged to take part in fighting because of the general summons of Jihad (fight in the cause of God) and yet shirked this duty. The reference here is to those who remained engrossed in personal concerns at a time when Jihad had become a collective obligation (fard bi al-kifayah). In the first case the person who fails to fight can only be a hypocrite, and God holds out no good promise for such a person unless there is good reason, for example, genuine disability. In the second case, however, what is required is the mobilization of a part rather than the entire military strength of the Islamic community. In such cases, if the recognized head (imam) of the Islamic community summons the people to come forward and undertake the expedition concerned, those who respond to that call are reckoned to be of superior merit to those who remain occupied with other pursuits however meritorious.
Fard bi al-kifayah signifies a collective duty of the Muslim community so that if some people carry it out no Muslim is considered blameworthy; but if no one carries it out all incur a collective guilt - Ed.