To answer that, Allah Ta'ala said: قُلْ يَوْمَ الْفَتْحِ لَا يَنفَعُ الَّذِينَ كَفَرُوا إِيمَانُهُمْ (Say, 'On the day of decision their belief will not be of any use to disbelievers,'-29) This amounts to saying: Why are you asking us about the day of our victory while that day is going to be a day of trouble for you? When victory comes to us, you would have already been overtaken by punishment. May be, right here in this world - as it happened in the battle of Badr - or, in the Hereafter. And when the punishment of Allah seizes someone, then, no profession of faith at that time, no declaration of 'Iman at that stage is accepted. (as mentioned by Ibn Kathir)
There are some others who have interpreted the 'day' in: مَتٰی ھٰذا الفَتح ("When will this decision take place?" ) as the day of Qiyamah. The summarized explanation of this verse in Bayan-ul-Qur'an appearing in the original edition of Ma` ariful-Qur'an is based on this tafsir. ( The translation of al-lath as 'decision' as given in the text admits both probablities)
Al-hamdulillah
The Commentary on
Surah As-Sajdah
Ends here.