"Those who believe, and those who followed Jewish law, and [also] Christians and Sabeans, whoever believes in Allah and in the Last Day and does good
deeds, they shall have their reward before their Lord, and there shall be no fear upon them, nor shall they grieve.” [Al-Baqarah]

Qadi al-Baydawi said the following in his respected exegesis (tafseer):
(Bracketed [] comments prefixed by SZ: are from Shaykh Zadeh’s exegetial annnotations on Baydawi’s exegesis. Parenthesised () comments are my own, added where the English translation of the author’s diction does not convey the meaning which is obvious in the original Arabic):

“‘Those who believe’ i.e. from the ummah of Muhammad. It has been said that the reference here is to the hypocrites, since they (i.e. ‘those who believe’ have been associated with the disbelievers (i.e. the other three groups - Jews, Christians and Sabeans are disbelievers, and thus mention of believers in the same category as them does not make sense, unless we take it as referring to [SZ: those who profess faith by their tongue only] and are therefore believers in the eyes of the world, but not before Allah)
. . . .
(he then discusses the etymological roots of the words for ‘Jewish’, ‘Christian’ and ‘Sabean’
. . . .
‘they shall have their reward before their Lord’, that is, whoever among them was following his religion before it was abrogated. [SZ: he (al-Baydawi) adds the word(s) ‘among them’ in consideration of the possibility that ‘whoever believes’ is a (part of speech in Arabic, signifying a particular type of subject) whose
(another part of speech, which refers to the predicate referring back to the subject) is ‘they shall have their reward’; and this predicate () along with its subject () comprises the predicate () for the words of The Exalted (meaning), “Those who believe . . .”. A phrasial invariably requires an <`aaid> (something which connects it to its , for otherwise we would have a hanging phrase, which is inadmissible), but none is (explicitly) mentioned, and so it (the <`aaid>) is implicitly understood to be ‘among them.’]”

Suheil adds: the above highlights the importance of being fully conversant with the Arabic language before attempting to interpret the Qur’an. In this light, the Prophet has said, “Whoever spoke about the Qur’an according to his (own, unlearned) opinion, or without knowledge, should seek his place in the Fire.”

From the above extract, we may also infer that there are two possibilities for interpreting the verse:

i) That there is an implicit <`aaid>, which renders the meaning as follows:
“Those who merely profess to be believers, as well as those who follow Jewish law, and Christians, and Sabeans - whoever among them now affirms faith in Allah and the Last Day (the hypocrites must do so by adding affirmation of their hearts to their verbal declaration of faith, the others by embracing the completion of revealed religion which was sent with Prophet Muhammad) shall have their reward before their Lord, and there shall be no fear upon them nor shall they grieve.”

ii) That the verse refers to those Jews, Christians, etc. who faithfully followed the religion revealed to them before it was abrogated. Thus, included in the promise of salvation given by this verse are those Jews who followed the Torah and died prior to the coming of Prophet Jesus, and those Christians who followed the Injeel
and did not commit and who died prior to the coming of Prophet Muhammad. This interpretation is supported by that which has been narrated by Ibn Kathir in his exegesis of the verse, which is that the verse was revealed about the former companions of Salman the Persian. Salman told the Prophet that “they used to pray and fast, and believe in you and that you would be coming as a prophet.”

Those Jews who rejected Jesus are excluded from the glad tidings, as evidenced by the verse (translated):
“. . . Jesus, Son of Mary, said to the disciples, ‘Who will be my supporters in the cause of Allah?’ The disciples said, ‘We are supporters of Allah(’s cause).’ So, a group of the Children of Israel believed and a group disbelieved.” [Last verse of Al-Saff]
Thus, those who rejected Jesus became disbelievers, and lost the promise of salvation. Similarly, those Jews and Christians who do not accept the message of Prophet Muhammad also become disbelievers, as testified to by the Hadith in Sahih Muslim,
“There is no Jew, nor any Christian, who hears about me, and then does not believe in me and in that with which I have been sent, except that Allah must enter him into the Fire.”

It is not permissible to interpret the verse as implying that all Jews, Christians and Sabeans will enter heaven, because:

a) This contradicts the proofs presented in ii) above (and other similar proofs which I have omitted above), and any interpretation of a verse which contradicts other decisive verses and ahadith is to be rejected.

b) It is not known with certainty who the Sabeans are.
Mujahid said: they are a people different from the Jews, Christians and Zoroastrians, and Ibn Kathir has preferred this view.
Abul-`Aliyah and al-Dahhak said: they are a sect of the People of the Book who read the Zabur.
Abu Ja`far al-Razi said: I have been informed that they worship the angels, recite the Zabur and pray to a Qiblah.
Wahb ibn Munabbih said: they are those who recognise Allah on their own, who have no shari`ah by which to act, and who do not commit kufr.
al-Qurtubi said: they claim that they follow the religion of Noah.
Some scholars said: they are those who did not receive the message of any prophet.
Abu Hanifah said: they are part of the People of the Book, who worship faced towards the planets.
Abu Yusuf and Muhammad ibn al-Hasan said: they worship the planets and believe the planets have an influence on them, and they are therefore disbelievers.

According to some of the above opinions, the Sabeans are clearly classified as disbelievers, who are not guaranteed salvation under any revelation. The absence of any decisive identification of the Sabeans brings this verse into the category of the - those verses whose exact meaning is ambiguous and should be left to
Allah.  A text whose exact meaning is vague or unclear cannot be used to prove a point even in jurisprudence, let alone in the fundamentals of doctrine. According to the self-evident rule of usul, the ambiguous must always be referred back to the decisive, and certainly, there is no dearth of decisive texts on the issue of salvation.
“As for those in whose hearts is a disease, they follow that of it which is ambiguous, seeking and seeking to interpret them whereas none knows their (exact) interpretation except for Allah.” [Aali-`Imran]

And Allah is the granter of success.