( يُبَشِّرُهُمْ رَبُّهُم بِرَحْمَةٍ مِّنْهُ…) (Their Lord gives them good tidings of mercy from Him): In addition to the victory they achieved in the Battle of Badr, they have another glad tiding, which is gaining the mercy and pleasure of Allah (swt).
This mercy is the basis for their entry into paradise because they do not enter paradise solely based on their deeds, struggles, or the expenditure of their wealth; rather, they enter it through the mercy of Allah (swt). This is clearly indicated by the Prophet (saws):
“None of you will enter Paradise by his deeds.” They said: Allah’s Messenger, not even you? Thereupon he said: Not even I, but that Allah should wrap me in His Mercy and Grace.“[1]
Just in making paradise a reward for those who strive and perform righteous deeds in this world, it is a mercy from Allah(swt).
(…وَرِضْوَانٍ…) (and the pleasure): When Allah (swt) is pleased with them, His displeasure will never follow thereafter. The Prophet (saws) said:
“Allah would say to the inmates of Paradise: O, Dwellers of Paradise, and they would say in response: At thy service and pleasure, our Lord, the good is in Thy Hand. He (the Lord) would say: Are you well pleased now? They would say: Why should we not be pleased, O Lord, when Thou hast given us what Thou hast not given to any of Thy creatures? He would, however, say: May I not give you (something) even more excellent than that? And they would say: O Lord, what thing can be more excellent than this? And He would say: I shall cause My pleasure to alight upon you and I shall never be afterwards annoyed with you.”[2]
(…وَجَنَّاتٍ لَّهُمْ فِيهَا نَعِيمٌ مُّقِيمٌ) (and of gardens for them wherein are enduring delights.): It is not a single paradise but multiple Paradises, wherein there is everlasting bliss. The enduring bliss is the permanent joy, unlike the transient pleasures of this world, where human existence is also transient. From birth, one is on a journey towards his end.
[1] Musnad Ahmad Bin Hanbal: Musnad Abi Hurairah (ra), Hadith no. 7473
[2] Sohih Bukhari: The Book of Riqaq, Chapter: The Description of Paradise and Hellfire, Hadith no. 6183