What is the response to the claim of multiple religions, considering that the Holy Qur’an referred only to one religion, which is Islam, and that Judaism and Christianity are but sects?

Religion is what Allah has legislated for His servants through His prophets and messengers. The prophets agree on matters of faith and monotheism, but they differ on laws and rulings, in accordance with His (the Almighty's) saying: "For each We have made from you a law and a clear way." The law of Prophet Muhammad, peace be upon him, superseded the previous laws, and it is "a guardian over it," meaning it abrogates and governs what came before it. The prophets are like brothers from different mothers, their father is one but their mothers are many. This means that the origin of their religion is one, even if its branches differ. It is permissible to refer to previous faiths as "religion" based on the belief that they originated from Allah, while acknowledging that their followers distorted them.