Allah stated that whoever follows what pleases Him, He will guide him to the paths of peace, that is Islam, or Paradise. Verily, Allah delivers His believing servants from the darkness of disbelief, doubt and hesitation, to the light of the plain, clear, explained, easy and unequivocal truth. He also stated that Shaytan is the supporter of the disbelievers who beautifies the paths of ignorance and misguidance that they follow, thus causing them to deviate from the true path into disbelief and wickedness.
أُولَـئِكَ أَصْحَـبُ النَّارِ هُمْ فِيهَا خَـلِدُونَ
(Those are the dwellers of the Fire, and they will abide therein forever.)
This is why Allah mentioned the light in the singular while mentioned the darkness in the plural, because truth is one, while disbelief comes as several types, all of which are false. Similarly, Allah said,
وَأَنَّ هَـذَا صِرَطِي مُسْتَقِيمًا فَاتَّبِعُوهُ وَلاَ تَتَّبِعُواْ السُّبُلَ فَتَفَرَّقَ بِكُمْ عَن سَبِيلِهِ ذَلِكُمْ وَصَّـكُمْ بِهِ لَعَلَّكُمْ تَتَّقُونَ
(And verily, this is my straight path, so follow it, and follow not (other) paths, for they will separate you away from His path. This He has ordained for you that you may have Taqwa) 6:153,
وَجَعَلَ الظُّلُمَـتِ وَالنُّورَ
(And originated the darknesses and the light) 6:1, and,
ظِلَـلُهُ عَنِ الْيَمِينِ
(to the right and to the lefts) 16: 48
There are many other Ayat on the subject that mention the truth in the singular and falsehood in the plural, because of falsehood's many divisions and branches.
Tafsir Ibn Kathir (abridged, English).