I must thank you for posing your question on this site in order to reach the truth and giving us the honor to answer your question. Taking some points in to consideration can help us reach the answer:

1- We believe that the Quran enjoys comprehensiveness, but such a claim does not mean we should expect the Quran to solve every single problem.

Even though there have been divergent understandings regarding the comprehensiveness of the Quran among Muslim scholars, but most of them understand it as an exhaustiveness that leads to fulfilling the aims of religion[1] and believe that the comprehensiveness of the Quran is defined within this field.

The Holy Quran is a book of guidance for all people and this is the sole purpose for its revelation. Therefore everything that is related to guidance is mentioned in it, whether it is truths pertaining to the genesis of this world, the Hereafter, ethical virtues, parables, rulings or admonitions that people need in order to find the right path.[2] We must keep in mind that religion is only responsible for explaining issues that people cannot understand or unveil on their own. In fact, the Holy Quran and generally Islam are not supposed to interfere in fields of knowledge that people can discover by themselves, because Allah has granted the human being mind, intellect and thought[3] and religion is definitely not aiming to take away these abilities of mankind, because this would mean that physical senses, intellect and human experience were created in vain.[4]  The Quran's primary teaching is understanding Allah and has deemed belief in the one God as the cornerstone of faith. After introducing Allah, understanding the Hereafter (believing in the Day of Judgment in which each person receives the rewards or punishments of his good or evil actions) is derived from it and it is considered another principle of faith. Subsequently understanding the Messengers of Allah is derived from understanding the Hereafter; because one is not rewarded or punished for his good or evil deeds prior to being informed of what acts are prohibited or obligatory through revelation and messengers. This too is considered another principle of faith, and as a result the Quran mentions the belief in one god, the belief in prophets and the belief in the Hereafter as the Usul al-Din (Fundamental Beliefs) of Islamic faith.

After that, it has explained the virtues and ethical features which are compatible with the three fundamental principles of faith that a believer must possess. Then it has mentioned and explained the practical rules that lead to true prosperity and salvation and helps one develop and enhance his ethical virtues, and greatest of all, develop the true beliefs of faith. Virtues are enhanced by a set of proper acts and conducts, and virtues do the same for fundamental beliefs; For example if one is proud, self-centered, selfish and egoistic, we should not expect him to believe in Allah and show humility for his Lord. Also one who has not understood fairness, care, compassion and mercy his whole life can not believe in the Day of Judgment.[5]

The Holy Quran fulfills man’s purpose of creation and does so in the most comprehensive way; because the aim of humanity, which is intertwined with realism, is one’s world view which necessitates carrying out the proper ethical principles and practical laws, and the Quran has assumed the responsibility of explaining these teachings in detail. When describing the Quran, the Almighty says: "یهدى الى الحق و الى طریق مستقیم"[6] "It guides to the truth (in belief) and to a straight path (in practice)."

2- The Quran does not differ from other holy books and is similar to them in terms of being a book of guidance; the only difference is that the Quran is greater than them.

In other words the Quran consists of the meaning of all previous holy books and everything that people are in need of to reach prosperity and salvation in the two fields of belief and practice.[7] From the Quran's point of view the true religion and faith was the same from the time of Adam up until the time of Muhammad. All Prophets, whether they were presenting a new religion or preaching the teachings of previous ones, have called humanity to the same ideology and faith. The Quran stresses this fact and says: " شرع لکم من الدین ماوصى به نوحاً و الذى اوحینا الیک و ما وصینا به ابراهیم و موسى و عیسى"[8]

"He has prescribed for you the religion which He had enjoined upon Noah and which We have [also] revealed to you, and which We had enjoined upon Abraham, Moses and Jesus"

The slight differences between holy religions are as follows:

A: Holy religions differed in a set of secondary issues that were dependant on time, circumstances, the environment and people's features. In other words they were all different shapes and parts of one truth and fulfilled the same goal.

B: As man's intellect progressed and developed, ensuing prophets presented their teachings, which were all in the same field, at a higher level.

This is the progression of religion, not the differing of religion, and this is why the previous prophets had glad tidings for the coming of next prophets while they would approve of and believe in the previous ones.

The Quran says: " و انزلنا الیک الکتاب بالحق مصدقاً لما بین یدیه من الکتاب و مهیمناً علیه "[9] " We have sent down to you the Book with the truth, confirming what was before it of the Book and as a guardian over it."

The word "مهیمن" means a guardian and keeper and the Quran being "Moheimen" over other holy books means:

First: The Quran maintains them and confirms their (holy books') truths.

Second: It protects them and is a guardian over them and mentions all the mistakes, distortions and alterations that have made their way into them[10]. This correction is definitely not limited to the field of practical rulings and Ahkam but it also covers fundamental beliefs.