It is obligatory to believe (in and acknowledge) everything stated by Allaah and His Messengers–Imaam Sadi (rahimahullaah)
Allaah (The Most High) said:
قُلۡ صَدَقَ ٱللَّهُۗ
”Say (O Muhammad): “Allah has spoken the truth.” [3: 95]
وَمَنۡ أَصۡدَقُ مِنَ ٱللَّهِ قِيلاً۬
”And whose words can be truer than those of Allah? (Of course, none).” [4: 122]
وَلَا يُنَبِّئُكَ مِثۡلُ خَبِيرٍ۬
”And none can inform you (O Muhammad) like Him Who is the All-Knower (of each and everything).” [35: 14]
قُلۡ أَىُّ شَىۡءٍ أَكۡبَرُ شَہَـٰدَةً۬ۖ قُلِ ٱللَّهُۖ
”Say (O Muhammad): “What thing is the most great in witness?” Say: “Allah (the Most Great!) is Witness between me and you.’‘[6:19]
لَّـٰكِنِ ٱللَّهُ يَشۡہَدُ بِمَآ أَنزَلَ إِلَيۡكَۖ أَنزَلَهُ ۥ بِعِلۡمِهِۦۖ وَٱلۡمَلَـٰٓٮِٕكَةُ يَشۡهَدُونَۚ وَكَفَىٰ بِٱللَّهِ شَہِيدًا
”But Allah bears witness to that which He has sent down (the Qur’an) unto you (O Muhammad), He has sent it down with His Knowledge, and the angels bear witness. And Allah is All-Sufficient as a Witness.’‘ [4:166]
شَهِدَ ٱللَّهُ أَنَّهُ ۥ لَآ إِلَـٰهَ إِلَّا هُوَ وَٱلۡمَلَـٰٓٮِٕكَةُ وَأُوْلُواْ ٱلۡعِلۡمِ قَآٮِٕمَۢا بِٱلۡقِسۡطِۚ لَآ إِلَـٰهَ إِلَّا هُوَ ٱلۡعَزِيزُ ٱلۡحَڪِيمُ
”Allah bears witness that La ilaha illa Huwa (none has the right to be worshipped but He), and the angels, and those having knowledge (also give this witness); (He is always) maintaining His creation in Justice. La ilah illa Huwa (none has the right to be worshipped but He), the All-Mighty, the All-Wise.” [3: 18]
The Aayaat (concerning) this mighty affair are many, and they are indications to the clearest proofs that the most obligatory of obligations upon the slaves is that they should believe and testify to everything Allaah (The Most High) has stated about Himself–regarding His perfect Attributes and that which He negated from Himself regarding attributes of deficiency, and that Allaah is more knowledgeable of that than His creation. His testimony is the greatest testimony, and what He has stated about Himself and everything else is of the highest degree of truthfulness. So this makes it obligatory on the slave that he (or she) does not enter the least of doubt in his (or her) heart regarding any information given by Allaah. He (or she) is to place that (information) in his (or her) as a firm creed which can neither be opposed by anything in opposition nor blemished by (any) doubt.
He (or she) should know with certain knowledge that it is not possible for anything to (rightfully) oppose the information given by Allaah and His Messenger, and everything that opposes and negates it-regardless what type of knowledge that is- is self-evident falsehood. (It is) false in its judgement and there is nothing to be (considered) sound that can be in opposition to what Allaah has stated.
[Also these Aayaat (3:95; 4:122; 35:14; 6:19; 4:166; 3: 18 etc)] indicate towards the greatest proof that the one who establishes his (or her) creed solely on the information given by Allaah and His Messenger, then indeed he (or she) has established it upon a strong foundation; rather he (or she) has established it on the foundation of all sound foundations even if opponents were appointed or decreed for it.
And how can this not be the case [i.e. that he (or she) is established upon the basis of all sound foundations] when it is the (fact that) the sound intellectual proofs, the sound innate uncorrupted human disposition, the sound uncorrupted paths of comprehension and the sound uncorrupted human (self, frame of mind etc) are all in support of the information given by Allaah and His Messengers—testifying to the truthfulness of that which Allaah and His Messengers have stated and its benefits. And because of this Allaah has praised the people of sound understanding amongst the creation because they establish their creed based on this foundation. (Allaah informed us that they say):
رَّبَّنَا إِنَّنَا سَمِعْنَا مُنَادِيًا يُنَادِي لِلْإِيمَانِ أَنْ آمِنُوا بِرَبِّكُمْ فَآمَنَّا ِ
“Our Lord! Verily, we have heard the call of one (Muhammad) calling to Faith: ‘Believe in your Lord,’ and we have believed.” [3: 193]
وَقَالُوا سَمِعْنَا وَأَطَعْنَا
”And they say, “We hear, and we obey.’‘ [2: 285)
الَّذِينَ يَسْتَمِعُونَ الْقَوْلَ فَيَتَّبِعُونَ أَحْسَنَهُ ۚ أُولَـٰئِكَ الَّذِينَ هَدَاهُمُ اللَّـهُ ۖ وَأُولَـٰئِكَ هُمْ أُولُو الْأَلْبَابِ
Those who listen to the Word [good advice La ilaha ill-Allah (none has the right to be worshipped but Allah) and Islamic Monotheism, etc.] and follow the best thereof (i.e. worship Allah Alone, repent to Him and avoid Taghut, etc.) those are (the ones) whom Allah has guided and those are men of understanding (like Zaid bin ‘Amr bin Nufail, Salman Al-Farisi and Abu Dhar Al-Ghifari). [39:18]
And from this one knows that the innovated statements and creeds of the futile people of philosophical rhetoric have neither been given any authority by Allaah nor are they based on the Qur’aan and the Sunnah; rather they are founded on the intellects of those whose mistakes and deviation is known. (Their views) are the most futile of all futilities and the most foolish of all foolish affairs because they turn away from the information given by Allaah and His Messengers—deceived by their souls that call to evil and abandoned to their intellects, which have neither been purified by the realities of Imaan nor have they been nourished with sound Imaan and firm certainty.
This foundation (i.e. belief and testifying to all the information given Allaah and His Messengers) is enough to refute all the statements of the people of deviation with a clear observation, whilst being acquainted with the falsehood (of such statements) in detail. That is because when we know that (these statements) are in opposition to the clear revealed Islamic texts and the sound unambiguous proofs transmitted by (the scholars from the Book, the Sunnah and the Salaf), then we will know their futility. And that is because anything that opposes the truth is falsehood and that which is in opposition to truthfulness is a lie.
[Source: Fat-hul Raheem Al-Malikil Allaam Fee Ilmil Aqaa-id Wat-Tawheed Wal Akhlaaq Wal-Ahkaam Al-Mustanbatu Minal Qur’aan’ page: 9-10. Abridged and slightly paraphrased]