Adopted from the book : "The Faith of Shi'a Islam" by : Allamah Muhammad Ridha al-Muzaffar"
Allah said:
Say (O Muhammad, unto mankind): I do not ask of a wage for this except love of my kinsfolk. (42;23)
We believe that in addition to the obligation to hold fast to the Household of the prophet, it is necessary for every Muslim, from the point of view of his din, to love them, for Allah, in this verse, has told mankind to love them.
It is narrated from the prophet:
Love of them (my Household) is a sign of belief, and to show enmity towards them is a sign of unbelief. Whosoever loves them, loves Allah and His Messenger. And whosoever shows enmity towards them shows enmity towards Allah and His Messenger.
Naturally we must love them; it is one of the necessary Islamic duties over which there can be no dispute or doubt. All the sects of Islam have ccepted this, apart from a few people who are recognized to be enemies of the Household of Prophet, and they have been given the derisive name of Nawasib3 (i.e those who have planted enmity of the Household in their hearts). They are counted among those who deny one of the necessary beliefs of Islam.
One who denies one of the Islamic commandments about which there is no doubt,such as the obligation to pray or to pay zakat, undoubtedly ranks with a denier of prophethood, even though he may utter the two testimonies of the Islamic faith (i.e ashhadu an la ilaha ila' llah wa ashhadu ana muhammadan rausl Allah. I testify that there is no god but Allah, and that Muhammad is the Messenger of Allah). Enmity towards Muhammad's Household is a sign of the unbelief, and love of them is a sign of faith. Therefore enmity towards is the same as enmity towards Allah and His Messenger.
Allah has ordered us to love the Imams, because they merit this, and because of their high degree of obedience to the commands of Allah their high position in the presence of Allah, their purity form polytheism, sin and that which keeps His servants away from the Mercy of Allah.
It is impossible that Allah should enjoin us to love someone who commits sin and does not obey Him as he should be obeyed; for all mankind are His slaves and created equally, and none have a special relationship or friendship with Him. The noblest of them in the sight of Allah is the best in conduct. So, if He instructs all people to love someone, that person must be the best among them in virtue and above them all, otherwise he would not deserve to be loved, and Allah would never prefer some person to another for no reason, or if that person had no merit.
Note :
3. This is not a particular group, but is used in general for anyone who insults or shows direspect for the family of the prophet