He is asking about the grammar of the word al-Saabi’oon and how we can refute those who say that it is a grammatical error in the Qur’aan
Question

I would like to know about the grammar of the word al-saabi’oon in Soorat al-Maa’idah. Why does it appear in the nominative form (al-saabi’oon ) when it appears in another soorah in the accusative (al-saabi’een ), although the syntax in both passages is very similar? This was the cause of a great argument between myself and a Christian person who says that there are grammatical mistakes in the Qur’aan. I told him that I would leave Islam if there was a single grammatical mistake in the Qur’aan. I said this out of strong faith and certainty that the Qur’aan is the word of Allaah, glorified and exalted be He far above what the fabricators say.
Praise be to Allah.The word al-Saabi’een  (in the accusative) appears in Soorat al-Baqarah and Soorat al-Hajj, where Allaah says (interpretation of the meaning): 
“Verily, those who believe and those who are Jews and Christians, and Sabians [wa’l-saabi’een ], whoever believes in Allaah and the Last Day and does righteous good deeds shall have their reward with their Lord, on them shall be no fear, nor shall they grieve”
[al-Baqarah 2:62] 
“Verily, those who believe (in Allaah and in His Messenger Muhammad), and those who are Jews, and the Sabians [wa’l-saabi’een], and the Christians, and the Majoos, and those who worship others besides Allaah; truly, Allaah will judge between them on the Day of Resurrection. Verily, Allaah is over all things a Witness”
[al-Hajj 22:17]
The same word appears in the nominative form in Soorat al-Maa’idah, where Allaah says (interpretation of the meaning): 
“Surely, those who believe (in the Oneness of Allaah, in His Messenger Muhammad and all that was revealed to him from Allaah), and those who are the Jews and the Sabians [wa’l-saabi’oon] and the Christians, — whosoever believed in Allaah and the Last Day, and worked righteousness, on them shall be no fear, nor shall they grieve”
[al-Maa’idah 5:69]
In the first two verses, there is nothing difficult about the grammar, because the word al-saabi’een follows the conjunction waw (and) is in agreement with the word alladheena (those who); it is the noun of the particle inna (translated here as “verily”) and as such appears in the accusative form of the sound masculine plural, in accordance with the rules of Arabic grammar. 
Where the confusion arises is in the third verse quoted, from Soorat al-Maa'idah, where the word appears in the same position with regard to word order, but appears in the nominative form.  
The grammarians and mufassireen explained this in several ways, and they mentioned several well-known similar usages in Arabic. It is sufficient here to quote just three of them, which are among the most well known. 
1 – The word order in the verse differs from everyday usage. Based on that, the meaning is that those who believe, and those who are the Jews and the Christians, whosoever believed in Allaah … on them shall be no fear, nor shall they grieve, and the same applies to the Sabians. So the subject appears in the nominative, as is indicated by the waw of the sound masculine plural. There is a similar example in Arabic verse where the poet says: 
Faman yaku amsa bi’l-madeenati rahlahu       fa inni wa qayyaarun biha la ghareeb
(Whoever ends up in Madeenah with his saddle, then Qayyaar and I are strangers). 
The point here is that the word Qayyaar – which is the name of his horse or camel – appears in the nominative here (qayyaarun) because it is the subject. It does not appear in the accusative even though it is preceded by the particle inna (inni = inna + the yaa (i) which is the accusative suffix representing the first person singular pronoun following the particle inna).   
2 – The word al-saabi’oon is the subject and the word al-nasaara (Christians) is in agreement with it. The phrase man aamana Billaah (whoever believed in Allaah) is the predicate of al-saabi’oon . The predicate of inna is omitted here, as is indicated by the predicate of the subject al-saabi’oon . A similar usage in Arabic appears in the line of verse: 
Nahnu bima ‘indina wa anta bima    ‘indika raadin wa’l-amru mukhtalif
(We with what we have and you with what you have are content, even though it is different). 
The point here is that the subject nahnu (we) is not followed by its predicate, because the predicate of anta (you) is sufficient. The predicate of anta – raadin (content) – includes the predicate of the first subject, nahnu (we). What these words mean is: we are content with what we have and you are content with what you have. 
3 – The word al-saabi’oon appears in conjunction with the word that takes the place of the noun of inna. If any of these particles – inna and its “sisters” – comes at the beginning of a nominal sentence that is composed of a subject and predicate, the noun of inna was originally nominative because it is the subject, before the word inna was introduced. Hence the word al-saabi’oon  is nominative because it is a word that takes the place of the noun of inna. 
See Awdah al-Masaalik by Ibn Hishaam, with a commentary by Muhiy al-Deen (1/352-366); and the Tafseer al-Shawkaani wa’l-Aloosi, on this verse. 
What you have mentioned, about your certain faith in the words of Allaah, is what is expected of every Muslim. Allaah says (interpretation of the meaning): 
“Do they not then consider the Qur’aan carefully? Had it been from other than Allaah, they would surely, have found therein many a contradiction”
[al-Nisa’ 4:82]
One of the things of which we must be certain is that this phrase, with this pronunciation, was revealed like this. This is how it was pronounced by the Prophet (peace and blessings of Allaah be upon him), this is how the Muslims learned it from him and recited it, and wrote it in the Mus-hafs, and they were pure Arabs. This became a new principle by means of which we learned a new usage of conjunctions in Arabic, even though it is not a common usage, but it is very eloquent and evocative… (End quote) 
Ibn ‘Aashoor tried to explain the eloquence of this word al-saabi’oon  appearing in the nominative. He said words to the effect that: the nominative in this context is unusual, so it makes the reader pause and wonder why this word is put in the nominative, when it would usually appear in the accusative.  
See the commentary on the verse from al-Maa'idah in Tafseer Ibn ‘Aashoor. 
For more information on who the Sabians were, see the answer to the question: “49048”. 
But there are a few points that should be noted from this discussion: 
Firstly: We should take an interest in shar’i knowledge; it is not sufficient to rely only on faith that we already have in our hearts even though that is the greatest source of protection.  If shar’i (Islamic) knowledge is added to that then – in sha Allaah – it will give extra protection against doubts and confusion that the enemies of our religion may try to stir up. 
Secondly: questions like this draw attention to the extent of negligence concerning one of the most important duties that we have towards the Book of Allaah, which is the duty to study and ponder it, not just recite it. Allaah says (interpretation of the meaning): 
“(This is) a Book (the Qur’aan) which We have sent down to you, full of blessings, that they may ponder over its Verses, and that men of understanding may remember”
[Saad 38:29]
Shaykh Ibn Sa’di (may Allaah have mercy on him) said: This is the reason why it was revealed, so that people may ponder its verses, derive knowledge from it and contemplate its meanings and rulings. For by studying it and contemplating its meanings, and studying it time after time, they will attain its blessings and goodness. This indicates that we are encouraged to study the Qur’aan, and that this is one of the best of deeds, and that reading that includes pondering the meanings is better than a quick recitation that does not achieve this purpose. The evidence for this is that if we were to undertake this obligation time after time, these verses would cause us to stop and wonder about the meaning, so that we would ask about it and research it, before we are confronted with specious arguments from our enemies. 
Thirdly: If we undertook the two obligations referred to above, we would be qualified to take the initiative and call others, telling them of the truth that we have and informing them – in the best manner – of the falseness of their ways, instead of being on the defensive, weak and defeated. And Allaah is the Source of strength.