Belief in the Books

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the previous rulings have been abrogated – i.e., they are no longer
applicable – and that new rulings and laws have been promulgated.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.