The people described here (2:4) are those whom Allah described in the preceding
    Ayah,
    ﴿الَّذِينَ يُؤْمِنُونَ بِالْغَيْبِ
        وَيُقِيمُونَ الصَّلوةَ وَمِمَّا رَزَقْنَـهُمْ يُنفِقُونَ ﴾
    (Those who have faith in the Ghayb and perform Salah, and spend out of what we have
    provided for them.)
    Mujahid once stated, "Four Ayat at the beginning of Surat Al-Baqarah describe the
    believers, two describe the disbelievers, and thirteen describe the hypocrites.''
    The four Ayat mentioned in this statement are general and include every believer,
    whether an Arab, non-Arab, or a person of a previous Scripture, whether they are
    Jinns or humans. All of these attributes complement each other and require the existence
    of the other attributes. For instance, it is not possible that one believes in the
    Unseen, performs the prayer and gives Zakah without believing in what the Messenger
    of Allah and the previous Messengers were sent with. The same with certainty in
    the Hereafter, this is not correct without that, for Allah has commanded the believers,
    ﴿يَـأَيُّهَا الَّذِينَ
        ءَامَنُواْ ءَامِنُواْ بِاللَّهِ وَرَسُولِهِ وَالْكِتَـبِ الَّذِى نَزَّلَ عَلَى رَسُولِهِ وَالْكِتَـبِ الَّذِى
        أَنَزلَ مِن قَبْلُ﴾
    (O you who believe! Believe in Allah, and His Messenger, and the Book (the Qur'an)
    which He has revealed to the Messenger, and the Book which He sent own to those
    before (him)) (4:136),
    ﴿وَلاَ تُجَـدِلُواْ أَهْلَ الْكِتَـبِ
        إِلاَّ بِالَّتِى هِىَ أَحْسَنُ إِلاَّ الَّذِينَ ظَلَمُواْ مِنْهُمْ وَقُولُواْ ءَامَنَّا
        بِالَّذِى أُنزِلَ إِلَيْنَا وَأُنزِلَ إِلَيْكُمْ وَإِلَـهُنَا وَإِلَـهُكُمْ وَاحِدٌ﴾
    (And argue not with the People of the Book, unless it be in (a way) that is better,
    except with such of them as do wrong; and say (to them): "We believe in that which
    has been revealed to us and revealed to you; our Ilah (God) and your Ilah (God)
    is One (i.e. Allah)') (29:46),
    ﴿يَـأَيُّهَآ الَّذِينَ
        أُوتُواْ الْكِتَـبَ
        ءَامِنُواْ بِمَا نَزَّلْنَا مُصَدِّقاً لِّمَا مَعَكُمْ﴾
    (O you who have been given the Book (Jews and Christians)! Believe in what We have
    revealed (to Muhammad ) confirming what is (already) with you) (4:47), and,
    ﴿قُلْ يَـأَهْلَ الْكِتَـبِ
        لَسْتُمْ عَلَى شَىْءٍ حَتَّى تُقِيمُواْ التَّوْرَاةَ وَالإِنجِيلَ وَمَآ أُنزِلَ
        إِلَيْكُمْ مِّن رَّبِّكُمْ﴾
    (Say (O Muhammad ): "O People of the Book (Jews and Christians)! You have nothing
    until you act according to the Tawrah (Torah), the Injil (Gospel), and what has
    (now) been revealed to you from your Lord (the Qur'an).'') (5:68).
    Also, Allah the Exalted described the believers;
    ﴿ءَامَنَ
        الرَّسُولُ بِمَآ  
            أُنزِلَ إِلَيْهِ مِن رَّبِّهِ وَالْمُؤْمِنُونَ كُلٌّ ءَامَنَ بِاللَّهِ وَمَلَ  ـئِكَتِهِ  وَكُتُبِهِ وَرُسُلِهِ لاَ نُفَرِّقُ بَيْنَ أَحَدٍ مِّن رُّسُلِهِ﴾
    (The Messenger (Muhammad ) believes in what has been revealed to him from his Lord,
    and (so do) the believers. Each one believes in Allah, His Angels, His Books, and
    His Messengers. (They say,) "We make no distinction between any of His Messengers'')
    (2: 285), and,
    ﴿وَالَّذِينَ ءَامَنُواْ بِاللَّهِ
        وَرُسُلِهِ وَلَمْ يُفَرِّقُواْ بَيْنَ أَحَدٍ مِّنْهُمْ﴾
    (And those who believe in Allah and His Messengers and make no distinction between
    any of them (Messengers)) (4:152),
    This is a sample of the Ayat that indicate that the true believers all believe in
    Allah, His Messengers and His Books.
    The faithful among the People of the Book, have a special significance here, since
    they believe in their Books and in all of the details related to that, so when such
    people embrace Islam and sincerely believe in the details of the religion, then
    they will get two rewards. As for the others, they can only believe in the previous
    religious teachings in a general way. For instance, the Prophet stated,
    «إِذَا حَدَثَكُمْ أَهْلُ الْكِتَابِ
        فَلَا تُكَذِّبُوهُمْ وَلَا تُصَدِقُوهُمْ وَلكِنْ قُولُوا: آمَنَّا بِالَّذِي أُنْزِلَ
        إِلَيْنَا وَأُنْزِلَ إِلَيْكُم»
    (When the People of the Book narrate to you, neither reject nor affirm what they
    say. Rather, say, 'We believe in what was revealed to us and what was revealed to
    you.')
    However, the faith that many Arabs have in the religion of Islam as it was revealed
    to Muhammad might be more complete, encompassing and firmer than the faith of the
    People of the Book who embraced Islam. Therefore, if the believers in Islam among
    the People of the Book gain two rewards, other Muslims who have firmer Islamic faith
    might gain an equal reward that compares to the two the People of the Book gain
    (upon embracing Islam). And Allah knows best.
    ﴿أُوْلَـئِكَ عَلَى هُدًى
        مِّن رَّبِّهِمْ وَأُوْلَـئِكَ هُمُ الْمُفْلِحُونَ ﴾
    (5. They are on guidance from their Lord, and they are the successful.)