The first reason is that as a result of the passage of time, and due to the fact that previous religions were not under the Divine protection of God, they underwent much change and variation. As a result, we see that the fundamental truths which were brought by all messengers now differ from one religion to another, the most apparent being the strict tenet of the belief and worship of God and God alone.
The second reason for this variation is that God, in His infinite Wisdom and eternal Will, decreed that all the divine missions prior to the final message of Islam brought by Muhammad, may the mercy and blessings of God be upon him, be limited to a specific time frame. As a result, their laws and methodologies dealt with the specific conditions of the people whom they had been sent to address.
Humanity has passed through numerous periods of guidance, misguidance, integrity, and deviation, from the most primitive age to the heights of civilization. Divine guidance accompanied humanity through all of this, always providing the appropriate solutions and remedies.
This was the essence of the disparity that existed between the different religions. This disagreement never went beyond the particulars of the Divine Law. Each manifestation of the Law addressed the particular problems of the people it was meant for. However, the areas of agreement were significant and many, such as fundamentals of faith; the basic principles and objectives of the Divine Law, such as protecting faith, life, reason, wealth, and lineage and establishing justice in the land; and certain fundamental prohibitions, some of the most important of these being idolatry, fornication, murder, theft, and giving false witness. Moreover, they also agreed upon moral virtues like honesty, justice, charity, kindness, chastity, righteousness, and mercy. These principles as well as others are permanent and lasting; they are the essence of all the Divine Messages and bind them all together.
