If outward resemblance in worldly affairs produces affection and alliance with them, then how much more so when the resemblance involves matters of religion? Such likeness all the more surely leads to a form of allegiance, and loving or siding with them stands in clear opposition to faith.

Allah, Exalted is He, says:

“O you who believe, do not take the Jews and the Christians as allies; they are allies only of one another. Whoever among you allies himself with them is indeed one of them. Truly, Allah does not guide the wrongdoing people. You see those in whose hearts is sickness rushing toward them, saying, ‘We fear that a turn of fortune may befall us.’ Yet it may well be that Allah will bring about victory or a command from Himself, and they will then regret what they have been concealing in their hearts. And those who believe will say, ‘Are these the ones who swore by Allah with their most solemn oaths that they were with you?’ Their works have come to nothing, and they are the losers.” (al-Māʾidah 5:51–53)

He, exalted be He, also says in reproach of the People of the Book:

“Those of the Children of Israel who disbelieved were cursed by the tongue of David and Jesus son of Mary. That was because they disobeyed and were habitually transgressing. They would not forbid one another the evil they committed—evil indeed was what they were doing. You see many of them taking the disbelievers as allies; evil indeed is that which their souls have sent ahead for them, for Allah is angry with them, and in the punishment they will abide forever. Had they believed in Allah, the Prophet, and what was sent down to him, they would not have taken them as allies; but most of them are defiantly disobedient.” (al-Māʾidah 5:78–81)

Thus the Almighty makes clear that believing in Allah, the Prophet, and what has been revealed to him necessarily entails refraining from allying with them. The presence of such allegiance, therefore, signifies the absence of faith, for the absence of what necessarily follows entails the absence of its prerequisite.

He, Most High, also says:

“You will not find a people who believe in Allah and the Last Day loving those who oppose Allah and His Messenger, even though they be their fathers, their sons, their brothers, or their kinsfolk. Such are the ones in whose hearts He has inscribed faith, and whom He has strengthened with a spirit from Himself.” (al-Mujādilah 58:22)