We always hear that there are religions; not just a single religion. What is your answer to this, knowing that the Quran refers to one sole religion when it mentions former and latter peoples, which is Islam?

All perfect praise be to Allaah, The Lord of the Worlds.  I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Religion is defined as the laws that were prescribed by Allaah The Almighty to His slaves and that were conveyed through His Prophets and Messengers. This includes all the rulings related to beliefs, morals and practical life. Therefore, all divine laws that were conveyed through the prophets, like Nooh (Nuh), Ibraaheem (Abraham), may Allaah exalt their mention, and our Prophet, Muhammad, sallallaahu ‘alayhi wa sallam, are called ‘religions’.

 All these religions shared two common factors: Faith and Tawheed (Islamic Monotheism), despite the fact that they differed in their legislation, methodology and rulings related to practical issues. Allaah The Almighty Says (what means): {So judge between them by what Allaah has revealed and do not follow their inclinations away from what has come to you of the truth. To each of you We prescribed a law and a method. Had Allaah willed, He would have made you one nation [united in religion], but [He intended] to test you in what He has given you; so race to [all that is] good. To Allaah is your return all together, and He will [then] inform you concerning that over which you used to differ.}[Quran 5:48] 

The Islamic Sharee‘ah (law) is the only valid Sharee‘ah and it has abrogated all previous Sharee‘ahs. Allaah The Almighty Says (what means): {And We have revealed to you, [O Muhammad], the Book in truth, confirming that which preceded it of the Scripture and as a criterion over it.}[Quran 5:48] That is to say, the Islamic Sharee‘ah has abrogated the Torah and the Gospel and other previous divine religions. The Prophet, sallallaahu ‘alayhi wa sallam, clarified that all religions from the previous prophets came from the same origin despite the fact that they differed in details. He clarified this through a wonderful example when he said that all prophets are half-brothers, having the same father but different mothers. [Muslim and Ahmad]

Hence, it is correct to call the laws that were conveyed by other prophets ’religions’ and to believe that they were sent from Allaah The Almighty. However, we must note that later on they were distorted and altered by the people they had been sent to.

Allaah Knows best.