The “seal of religions” is the final “mursal” religion. Meaning that after such a religion, it isn't possible for another messenger or religion to be sent.
One of the conditions of a religion being the seal of religions, is for it to be kamal religion. Another condition is for it to be immune to falsification.
The “seal of religions” is the final “mursal” religion, meaning that after it, it isn't possible for any other messenger or religion to be sent. As a result, the seal of religions, and in other words, the final “mursal” religion, whose prophet is the highest of all prophets and belongs to all people from the time of its revelation till the Day of Judgment, must be the most complete of all religions and must contain all of the elements of the “nafsul-amr” religion that need to be expressed through revelation. The only elements it might not to bear, are the ones that the human intellect can figure out and reach on its own without the help of revelation. According to this, being the most complete religion doesn’t mean that such a religion has the answers to every question related to it, such that anyone who doesn’t find the answers to his/her religious questions when referring to it, can conclude that the religion isn't a complete one.
Therefore, everything that is meant to be sent to man through revelation, can be found in the seal of religions and by revealing such a religion, the series of mursal religions will end, and no other ones will ever be sent afterwards.
One of the conditions of a religion being the seal of religions, is for it to be the complete religion. Another condition is for it to be immune to falsification. Such an objective has been achieved through two ways regarding Islam:
1- Its most important religious source, the Quran, is one immune to falsification.
2- The “key method” of making use of this religious source also exists, which is the method of its interpretation by the Ahlul-Bayt that has been passed down generation after generation by their students until it has reached us and manifested in traditional ijtihad and Jawaheri fiqh.
What is meant by the “key method” is that this method is one used to evaluate other ones. Therefore, this method is a set of unchanging principles and rules that perfect over time in compliance with the needs of every era, without any of the principles experiencing any change whatsoever.
For further information, see:
Mahdi Hadavi Tehrani, Velayat va Diyanat, The Cultural