Belief in the Books implies four things:
1 – Firm belief that all of them were revealed from Allah, and that Allah spoke them in a real sense. Some of them were heard from Him from behind a veil, without there being any intermediary from among the angels. Some were conveyed by an angelic Messenger to a human Messenger. Some were written by Allah’s own Hand, as He says (interpretation of the meaning):
“It is not given to any human being that Allah should speak to him unless (it be) by Revelation, or from behind a veil, or (that) He sends a Messenger to reveal what He wills by His Leave. Verily, He is Most High, Most Wise.” [42:51]
“… and to Musa Allah spoke directly.” [4:164]
And Allah said concerning the Tawrat:
“And We wrote for him on the Tablets the lesson to be drawn from all things and the explanation for all things.” [7:145]
2 – The Books that Allah has mentioned by name, we must believe in by name. These are the Books which Allah has named in the Quran, i.e., the Quran, the Tawrat, the Injeel (Gospel), the Zaboor (Psalms), and the Scriptures of Ibrahim and Musa.
Those which Allah has mentioned in general terms, we must believe in them in general terms, so we say concerning them what Allah and His Messenger have commanded us to say:
“…say: “I believe in whatsoever Allah has sent down of the Book [all the holy Books…].” [42:15 – interpretation of the meaning]
3 – Believing whatever is true of what they say, such as what is said in the Quran, and whatever has not been altered or distorted in the previous Books.
4 – Believing that Allah revealed the Quran as a witness over these Books and to confirm them, as Allah says (interpretation of the meaning):
“And We have sent down to you (O Muhammad) the Book (this Quran) in truth, confirming the Scripture that came before it and Muhaymin (trustworthy in highness and a witness) over it (old Scriptures).” [5:48]
The mufassirin (commentators) said that Muhaymin means trustworthy and a witness over the Books that came before it, and confirming them, i.e., confirming whatever is true in them and rejecting whatever alterations, distortions and changes have occurred in them, and ruling that the previous rulings have been abrogated – i.e., they are no longer applicable – and that new rulings and laws have been promulgated. Hence all those who were following previous books and who have not turned on their heels should submit to it, as Allah says (interpretation of the meaning):
“And when it is recited to them, they say: We believe in it. Verily, it is the truth from our Lord. Indeed even before it we have been from those who submit themselves to Allah in Islam as Muslims.” [28:52-53]
It is obligatory for all the ummah (followers of Muhammad) to follow the Quran both outwardly and inwardly, and to adhere to it, and give it its due, as Allah says (interpretation of the meaning):
“And this is a blessed Book (the Quran) which We have sent down, so follow it and fear Allah.” [6:155]
What is meant by adhering to the Quran and giving it its due is: regarding what it permits as permitted, regarding what it forbids as forbidden, obeying its commands, avoiding that which it prohibits, paying heed to its lessons, learning from its stories, understanding its clear and unambiguous texts, referring the exact meaning of its ambiguous texts to Allah, not overstepping its limits and defending it, whilst memorizing it, reciting it in a proper manner, pondering the meanings of its verses, reciting it in prayer night and day, being sincere towards it in all senses, and calling others to do likewise with understanding.
This belief will bear great fruit for a person, the most important of which is:
1- Knowledge of Allah’s care for His slaves, for He has revealed to each people a Book by which they may be guided.
2- Knowledge of the wisdom of Allah in the laws which He has set out for each people, laws which are suited to their circumstances as Allah says (interpretation of the meaning): “To each among you, We have prescribed a law and a clear way.” [5:48].
3- Fulfilling the duty of giving thanks to Allah for this great blessing.
4- The importance of paying attention to the Quran, by reading it, pondering its meanings, understanding its meanings and acting accordingly.
And Allah knows best.
See A’alam al-Sunnah al-Manshoorah; Sharh al-Usool al-Thalathah by Shaykh Ibn ‘Uthaymeen.