In The Name of Allaah, The Most Merciful, The Bestower of Mercy.

Imaam As-Sadi [may Allaah have mercy upon him] said: It (i.e. Islam) is the mighty comprehensive religion, which commands belief in every (divine) book that Allaah sent down, and in every Messenger sent by Allaah. Allaah said:

فلذلك فادع واستقم كما أمرت ولا تتبع أهواءهم وقل آمنت بما أنزل الله من كتاب وأمرت لأعدل بينكم الله ربنا وربكم لنا أعمالنا ولكم أعمالكم لا حجة بيننا وبينكم الله يجمع بيننا وإليه المصير

So unto this (religion of Islam, alone and this Qur’an) then invite (people) (O Muhammad ), and Istaqim [(i.e. stand firm and straight on Islamic Monotheism by performing all that is ordained by Allah (good deeds, etc.), and by abstaining from all that is forbidden by Allah (sins and evil deeds, etc.)], as you are commanded, and follow not their desires, but say, “I believe in whatsoever Allah has sent down of the Book [all the divine Books – the Taurat (Torah), the Injeel or the Pages of Ibrahim (Abraham), and this Qur’an as the final one, and the only one to be acted upon], and I am commanded to do justice among you, Allah is our Lord and your Lord . For us our deeds and for you your deeds (good and bad). There is no dispute between us and you [i.e. after the realities have been manifested, and after the clear distinction between truth and falsehood, and after the clear distinction between guidance and misguidance, there is no room for debate, because the goal behind debate is to distinguish between right and wrong, so that the one to be guided is guided and the proofs established against the misguided one (see tafseer Sadi)]. Allah will assemble us (all), and to Him is the final return. [Surah Ash-Shooraa. Aayah 15]

Commentary: This (i.e. belief in all the books and Messengers) is contrary to the belief of the person who believes in some of the divine books and disbelieves in some, or believes in some of the Messengers and disbelieves in some. As for the religion of Islam, it commands one to believe in all the divine books. Allaah says: [وقل آمنت بما أنزل الله من كتاب – [all the divine Books – the Taurat (Torah), the Injeel or the Pages of Ibrahim (Abraham), and this Qur’an as the final one, and the only one to be acted upon]- Meaning, belief in every book that was revealed to any Messenger, even the books whose names are unknown or nothing is known regarding their details; but we believe in them and that they were books of truth, justice, wisdom, mercy and a source of upright rectification for the people they were revealed to; and that whoever believed in them was successful and those who disbelieved in them incurred total loss. We believe that they were revelation from Allaah sent down to His Messengers. We also believe in all the Messengers. Allaah said:

[كُلٌّ ءَامَنَ بِٱللَّهِ وَمَلَـٰٓٮِٕكَتِهِۦ وَكُتُبِهِۦ وَرُسُلِهِۦ لَا نُفَرِّقُ بَيۡنَ أَحَدٍ۬ مِّن رُّسُلِهِۦ‌ۚ – Each one (i.e. the Messenger Muhammad and the believers) believes in Allah, His Angels, His Books, and His Messengers. They say, “We do not make any distinction between one another of His (Allaah’s) Messengers (i.e. by believing in some and disbelieving in some)]. [From 22mins 28secs to 25mins 05secs. paraphrased. https://www.al-badr.net/detail/8LD6UgwQrZ ]