As a bounty from your Lord!
That will be the supreme success!
meaning, that will be from His bounty and kindness towards them.
It was reported in the Two Sahihs that the Messenger of Allah said:
اعْمَلُوا وَسَدِّدُوا وَقَارِبُوا وَاعْلَمُوا أَنَّ أَحَدًا لَنْ يُدْخِلَهُ عَمَلُهُ الْجَنَّة
Work and strive hard, and know that no one will enter Paradise by virtue of his deeds.
They said, "Not even you, O Messenger of Allah?"
He said,
وَلاَ أَنَا إِلاَّ أَنْ يَتَغَمَّدَنِيَ اللهُ بِرَحْمَةٍ مِنْهُ وَفَضْل
Not even me, unless Allah showers me with His mercy and grace.
فَإِنَّمَا يَسَّرْنَاهُ بِلِسَانِكَ لَعَلَّهُمْ يَتَذَكَّرُونَ