The lightning almost snatches away their sight. Every time it lights [the way] for them, they walk therein; but when darkness comes over them, they stand [still]. And if Allāh had willed, He could have taken away their hearing and their sight. Indeed, Allāh is over all things competent.
This is another parable which Allah gave about the hypocrites who sometimes know the truth and doubt it at other times. When they suffer from doubt, confusion and disbelief, their hearts are,
كَصَيِّبٍ
(Like a Sayyib), meaning, "The rain", as Ibn Mas`ud, Ibn `Abbas, and several other Companions have confirmed as well as Abu Al-`Aliyah, Mujahid, Sa`id bin Jubayr, `Ata', Al-Hasan Al-Basri, Qatadah, `Atiyah Al-`Awfi, `Ata' Al-Khurasani, As-Suddi and Ar-Rabi` bin Anas. Ad-Dahhak said "It is the clouds." However, the most accepted opinion is that it means the rain that comes down during,
ظُلُمَـتِ
(darkness), meaning, here, the doubts, disbelief and hypocrisy.
The last four verses bring out the miserable plight of the hypocrites with the help of two extended similes. The choice of two examples is meant to divide the hypocrites into two kinds of men. On the one hand were those in whom disbelief had taken deep roots, so that they had little inclination towards Islam, but pretended to be Muslims for worldly motives - the Holy Qur'an compares them to the man who, having found light, again loses it, and is left in darkness. On the other hand were those who did recognize the truth of Islam, and sometimes wished to be genuine Muslims, but worldly interests would not allow them to do so, and they remained in a perpetual state of hesitation and doubt - they have been likened to the men caught in a thunderstorm who move forward a step or two when there is a flash of lightning, but, when it is over, again get stuck. In the course of these parables, the hypocrites have also been warned that they are not beyond the power of Allah, and that He can, as and when He likes, take away their sight and hearing, and even destroy them.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 20–23
Fear is another factor which prevents them from acknowledging the truth. Rain is a wonderful blessing of God. But with rain, come thunder and lightning, which terrify faint-hearted people. God’s word as revealed to His prophets affects them similarly. On the one hand, it holds out to man the promise of great success, but on the other hand, its acceptance places man temporarily at risk: there is the fear of losing one’s worldly position; of clashing with the conventional set-up. Fear of these things sometimes makes one halt, and sometimes makes one move hesitantly on. But such caution cannot help one. Those who do not respond to God’s call open-heartedly shall be deprived of God’s blessings in this world as well as in the Hereafter.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.