Believing in the books of Allah that He sent down to His Messengers (peace be upon them) is an important matter and one of the pillars of faith without which true faith is not attained. Ibn Taymiyah said, ‘If someone believes in all the Books but disbelieves in one Book, then he is a disbeliever, unless he believes in that Book too.’ [4] See: Majmu‘ al-Fatawa, 1/371.

This is indicated by the Qur’an the Sunnah and scholarly consensus (ijma‘):
2.1. Evidence from the Qur’an:
i. Allah, may He be exalted, says, "O you who have believed, believe in Allah and His Messenger and the Book that He sent down upon His Messenger and the Scripture which He sent down before. And whoever disbelieves in Allah, His angels, His books, His messengers, and the Last Day has certainly gone far astray." (al-Nisa’ 4:136) Ibn Jarir said, ‘No person’s faith is valid unless he believes in that in which Allah has commanded him to believe; disbelieving in any part of it is like disbelieving in and rejecting all of it.’ [5] See: Tafsir Ibn Jarir, 4/594-596.

ii. Allah, may He be exalted, says, "Say, O believers, “We have believed in Allah and what has been revealed to us and what has been revealed to Abraham and Ishmael and Isaac and Jacob and the Descendants and what was given to Moses and Jesus and what was given to the prophets from their Lord. We make no distinction between any of them, and we are Muslims in submission to Him.” (al-Baqarah 2:136) As-Sa‘di said, This indicates that we should believe in all the Books that were revealed to all the Prophets. [6] See: Tafsir al-Sa‘di, p. 68.

2.2. Evidence from the Sunnah
This includes the report narrated from ‘Umar ibn al-Khattab (may Allah be pleased with him), according to which Jibril (peace be upon him) asked the Prophet (blessings and peace of Allah be upon him) about faith, and he said, “It is to believe in Allah, His angels, His Books, His Messengers, the Last Day, and to believe in al-qadar (the divine will and decree), both good and bad.” [7] Narrated by Muslim (8) in a lengthy hadith.

Abu’l-‘Abbas al-Qurtubi said about belief in the Books of Allah, ‘It means believing that  they are the words of Allah and came from Him, that what they contain is true, and that Allah, may He be exalted, has instructed His creation to follow their rulings and seek to understand their meanings by way of showing devotion to Him.’ [8] See: al-Mufhim, 1/145.

Believing in the divinely-revealed Books is a matter concerning which there is consensus that it is obligatory, and the one who does not do that is a disbeliever
Ibn Battah said, ‘If a man believes in everything that the Messengers brought except one thing, by rejecting that thing he becomes a disbeliever, according to all the scholars.’ [9] See: al-Inabah al-Sughra, p. 232.

Ibn Taymiyah said, ‘The Muslims are unanimously agreed that as part of what is well established in Islamic teaching and no one has any excuse for not knowing it, it is obligatory to believe in all the Prophets and Messengers, and in all the Books that Allah has sent down.’ [10] See: al-Jawab al-Sahih, 3/371.

The rational evidence for it being obligatory to believe in the Books includes the following [11] See: Abu Bakr al-Jaza’iri: Minhaj al-Muslim, p. 21.

i. Humans are vulnerable and need the help of their Lord to maintain their physical and spiritual well-being. This means that it was necessary to send down Books containing laws and regulations which enable them to attain levels of perfection and guide them to what they need in this world and the hereafter.

ii. Because the Messengers are the intermediaries between Allah the Creator, may He be exalted, and His creation, and the Messengers were like other humans, who live for a short time then die, if their messages were not contained in specific Books, then those messages would die when they died, and people would be left without a message after they were gone. Thus the original purpose of the revelation and the message of the messengers would be lost. It is for this reason that it was necessary for the divinely-revealed Books to be sent down.