The Order to Have Faith after Believing
Allah commands His faithful servants to adhere to all the elements of faith, its branches, pillars and cornerstones. This is not stated as mere redundancy, but from the view of completing faith and the continual maintenence of it. For instance, the believer proclaims in every prayer,
اهْدِنَا الصِّرَاطَ الْمُسْتَقِيمَ
(Guide us to the straight way.) which means, make us aware of the straight path and increase us in guidance and strengthen us on it. In this Ayah 4:136, Allah commands the believers to believe in Him and in His Messenger, just as He said elsewhere,
يأَيُّهَا الَّذِينَ ءَامَنُواْ اتَّقُواْ اللَّهَ وَءَامِنُواْ بِرَسُولِهِ
(O you who believe! Have Taqwa of Allah, and believe in His Messenger, ). Allah's statement,
وَالْكِتَـبِ الَّذِى نَزَّلَ عَلَى رَسُولِهِ
(and the Book which He has sent down to His Messenger,) refers to the Qur'an, while,
وَالْكِتَـبِ الَّذِى أَنَزلَ مِن قَبْلُ
(and the Scripture which He sent down to those before (him);) refers to the previously revealed divine Books. Allah then said,
وَمَن يَكْفُرْ بِاللَّهِ وَمَلَـئِكَتِهِ وَكُتُبِهِ وَرُسُلِهِ وَالْيَوْمِ الاٌّخِرِ فَقَدْ ضَلَّ ضَلَـلاً بَعِيداً
(and whosoever disbelieves in Allah, His Angels, His Books, His Messengers, and the Last Day, then indeed he has strayed far away.) meaning, he will have deviated from the correct guidance and strayed far away from its path.
إِنَّ الَّذِينَ ءَامَنُواْ ثُمَّ كَفَرُواْ ثُمَّ ءَامَنُواْ ثُمَّ كَفَرُواْ ثُمَّ ازْدَادُواْ كُفْراً لَّمْ يَكُنْ اللَّهُ لِيَغْفِرَ لَهُمْ وَلاَ لِيَهْدِيَهُمْ سَبِيلاً - بَشِّرِ الْمُنَـفِقِينَ بِأَنَّ لَهُمْ عَذَاباً أَلِيماً
Tafsir Ibn Kathir (abridged, English).