What is the reason for the significant differences between the monotheistic religions in their foundations and pillars, despite knowing that the religion with Allah is Islam and that all prophets were Muslims?

Islam has a general meaning that encompasses submission to God, obedience to Him, and following His messengers, and it is the religion of all prophets. It also has a specific meaning related to the message of the Prophet Muhammad, peace be upon him, and his law. The foundation of religion is the monotheistic worship of God and turning one's face to Him, while matters of lawful and unlawful are branches of this foundation. The divine laws agreed upon their general principles and fundamentals, and the differences in branches are due to the distortion by the People of the Book. No explicit text in the Gospels prohibits polygyny, and it has been historically proven that some early Christians married more than one woman. All prophets shared one religion, which is the worship of God alone, even if their laws and methodologies differed.