Commentary
In the opening verse (12), قل لِّمَن مَّا فِي السَّمَاوَاتِ the disbelievers have been asked as to who is the Master of the entire universe and all that it contains. Then, Allah Himself gives the answer through the blessed words of the Holy Prophet ﷺ that Allah is the Master of all. The reason for answering the question, rather than waiting for the disbelievers to answer it, is that the answer given was an accepted fact with the disbelievers of Makkah as well, for they were, though involved with Shirk and idolatry, no deniers of the fact that Allah Al-mighty was the Master of the heavens and the earth and everything they contained.
The word, اِلٰی :ila (towards) in the next sentence: لَيَجْمَعَنَّكُمْ إِلَىٰ يَوْمِ الْقِيَامَةِ He will surely gather you towards a day of doom) has been used either in the sense of tawaffa meaning to take in full, bring to a finish, or gather, (as in the translation of the meaning given here), in which case, it would mean that Allah Ta` ala will gather everyone, from the beginning to the end, on the day of Qiyamah - or, it could mean ` to gather in the graves,' in which case, it would mean that human beings will be kept being gathered into their resting places unto the day of Qiyamah when they will be raised back to life. (Qurtubi)
As for the sentence: كَتَبَ عَلَىٰ نَفْسِهِ الرَّحْمَةَ (He has prescribed for Himself to be merciful) appearing a little earlier in verse 12, a narration from Sayyidna Abu Hurairah ؓ in the Sahih of Muslim reports that the Holy Prophet ﷺ said: When Allah Ta` la made His creation, He made a promise to Himself in writing which is with Him, written in which are the words: اِنَّ رَحمَتِی تَغلِبُ عَلٰی غَضَبِی that is, ` My mercy shall remain dominant over My wrath.' (Qurtubi)
The sentence which appears at the end of the verse: الَّذِينَ خَسِرُوا أَنفُسَهُمْ (Those who have brought loss to themselves ... ) indicates that the deprivation of the disbelievers from the universal mercy of Allah Almighty mentioned earlier was caused by their own deeds, for they did not take to the way which brings mercy - that is, the way of belief and faith. (Qurtubi)