Faith and doing Righteous Deeds equals Salvation in all Times
Allah says;
إِنَّ الَّذِينَ امَنُواْ وَالَّذِينَ هَادُواْ وَالنَّصَارَى وَالصَّابِيِينَ مَنْ امَنَ بِاللَّهِ وَالْيَوْمِ الاخِرِ وَعَمِلَ صَالِحاً فَلَهُمْ أَجْرُهُمْ عِندَ رَبِّهِمْ وَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ
Verily, those who believe and those who are Jews and Christians, and Sabians (Sabi'in), whoever believes in Allah 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.
After Allah described the condition - and punishment - of those who defy His commands, fall into His prohibitions and transgress set limits by committing prohibited acts, He stated that the earlier nations who were righteous and obedient received the rewards for their good deeds. This shall be the case, until the Day of Judgment. Therefore, whoever follows the unlettered Messenger and Prophet shall acquire eternal happiness and shall neither fear from what will happen in the future nor become sad for what has been lost in the past.
Similarly, Allah said,
أَلا إِنَّ أَوْلِيَأءَ اللَّهِ لَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ
No doubt! Verily, the Awliya of Allah, no fear shall come upon them nor shall they grieve. (10;62)
The angels will proclaim to the dying believers, as mentioned,
إِنَّ الَّذِينَ قَالُواْ رَبُّنَا اللَّهُ ثُمَّ اسْتَقَـمُواْ تَتَنَزَّلُ عَلَيْهِمُ الْمَلَـيِكَةُ أَلاَّ تَخَافُواْ وَلَا تَحْزَنُواْ وَأَبْشِرُواْ بِالْجَنَّةِ الَّتِى كُنتُمْ تُوعَدُونَ
Verily, those who say;"Our Lord is Allah (alone)," and then they stand firm, on them the angels will descend (at the time of their death) (saying);"Fear not, nor grieve! But receive the glad tidings of Paradise which you have been promised!" (41;30)
The Meaning of Mu'min, or Believer
Ali bin Abi Talhah narrated from Ibn Abbas, about,
إِنَّ الَّذِينَ امَنُواْ وَالَّذِينَ هَادُواْ وَالنَّصَارَى وَالصَّابِيِينَ مَنْ امَنَ بِاللَّهِ وَالْيَوْمِ الاخِرِ
(Verily, those who believe and those who are Jews and Christians, and Sabians, whoever believes in Allah and the Last Day) that Allah revealed the following Ayah afterwards,
وَمَن يَبْتَغِ غَيْرَ الاِسْلَـمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِى الاٌّخِرَةِ مِنَ الْخَـسِرِينَ
And whoever seeks religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers. (3;85).
This statement by Ibn Abbas indicates that Allah does not accept any deed or work from anyone, unless it conforms to the Law of Muhammad that is, after Allah sent Muhammad. Before that, every person who followed the guidance of his own Prophet was on the correct path, following the correct guidance and was saved.
Why the Jews were called `Yahud
The Jews are the followers of Prophet Musa, who used to refer to the Tawrah for judgment. Yahud is a word that means,
`repenting', just as Musa said,
إِنَّا هُدْنَـا إِلَيْكَ
(We have Hudna (we repent, go back and return) unto You), (7;156).
Why the Christians were called Nasara
Allah said;
مَنْ أَنصَارِى إِلَى اللَّهِ قَالَ الْحَوَارِيُّونَ نَحْنُ أَنْصَارُ اللَّهِ
"Who will be my helpers in Allah's cause" Al-Hawariyyun said;"We are the helpers of Allah." (61;14)
It was said that;
they were called `Nasara', because they inhabited a land called An-Nasirah (Nazareth), as Qatadah, Ibn Jurayj and Ibn Abbas were reported to have said, Allah knows best.
Nasara is certainly plural for Nasran.
When Allah sent Muhammad as the Last and Final Prophet and Messenger to all of the Children of Adam, mankind was required to believe in him, obey him and refrain from what he prohibited them; those who do this are true believers. The Ummah of Muhammad was called `Mu'minin' (believers), because of the depth of their faith and certainty, and because they believe in all of the previous Prophets and matters of the Unseen.
The Sabi'un or Sabians
There is a difference of opinion over the identity of the Sabians.
Sufyan Ath-Thawri said that Layth bin Abu Sulaym said that Mujahid said that,
"The Sabians are between the Majus, the Jews and the Christians. They do not have a specific religion."
Similar is reported from Ibn Abi Najih.
Similar statements were attributed to Ata and Sa`id bin Jubayr.
They (others) say that;
the Sabians are a sect among the People of the Book who used to read the Zabur (Psalms), others say that they are a people who worshipped the angels or the stars.
It appears that the closest opinion to the truth, and Allah knows best, is Mujahid's statement and those who agree with him like Wahb bin Munabbih, that;
the Sabians are neither Jews nor Christians nor Majus nor polytheists. Rather, they did not have a specific religion that they followed and enforced, because they remained living according to their Fitrah (instinctual nature). This is why the idolators used to call whoever embraced Islam a `Sabi', meaning, that he abandoned all religions that existed on the earth.
Some scholars stated that;
the Sabians are those who never received a message by any Prophet.
And Allah knows best