What is the wisdom behind the multiplicity of heavenly religions, and why was religion not revealed completely to humanity from the beginning?

Allah is not questioned about what He does, due to His ultimate wisdom. The religion of Allah has been one since the beginning: it is submission to Allah through monotheism, faith, and obedience. This is the religion of all prophets. As for the differences in the details of lawful and unlawful across different laws, this is from Allah's mercy and justice. He legislated for each nation what suited it, with everyone agreeing on the major principles, until the Prophet Muhammad, peace be upon him, was sent with a lasting law until the Day of Judgment.