Imām Abū Jaʿfar Muḥammad ibn Jarīr al-Ṭabarī [d. 310 AH] said:

The elucidated meaning from the name of Allāh al-Raḥmān (The Most Merciful) is different from that of the name al-Raḥīm (The Giver of Mercy). For His name al-Raḥmān denotes all-encompassing mercy for His entire creation while His name al-Raḥīm stipulates mercy for some of His creation, either corresponding to all of their conditions, or some of them. There then should be no doubt—providing this opinion is correct—that the specificity of His name al-Raḥīm is continuously applicable, whether it be in this dunyā or the ākhirah, or both. If what we concur in this matter is correct, and Allāh Glorified in Praise has specifically and expeditiously shown tenderness towards the believers in this dunyā through granting them knowledgeable, actionable, guidance towards executing acts of obedience to Him, along with believing in His messengers, following His commands, refraining from acts of disobedience to Him as opposed to the likes of the abandoned ones who have chosen to associate partners in worship with Him, disbelieving [in Him], behaving in a contrary manner to His orders, and perpetrating acts of disobedience to Him; while realising that He, the Exalted in Praise, has prepared in the ākhirah for the ones who believe in Him, testify as to the truthfulness of His messengers, and execute acts of obedience with sincerity, jannah containing unimaginable, endless bounties and a much evident success. It should be unambiguous then that Allāh has specifically granted His mercy to the believers in [both] this dunyā and the ākhirah in addition to what they share with the disbelievers in this dunyā from [general] bounties and goodness that encompasses all of them including the availability of provisions, the subjugation of rain clouds that bring forth vegetation, the robustness of their minds and bodies, and other than that from bounties that are above enumeration, concerning which the believers share with those who do not believe.

Thus, our Lord the Exalted in Praise, is merciful to all of His creation in this dunyā and the ākhirah, but has a specific type of mercy for the believers that is manifested in this dunyā and in the ākhirah.

As for His mercy that encompasses all [of His creation] in this dunyā, He is al-Raḥmān in relation to it. This is inclusive of what we have mentioned and other than it for which there exists no avenue towards truly encompassing all of it for any of His creation. As He, the Exalted in Praise, says:

وَإِن تَعُدُّوا نِعْمَةَ اللَّهِ لَا تُحْصُوهَا ‎﴿١٨﴾‏

‘And if you should count the favours of Allāh, you could not enumerate them.’
[Al-Naḥl, 16:18]

Concerning the ākhirah, He has also encompassed His entire creation with His mercy for which He is al-Raḥmān. For He, the Exalted in Remembrance, has proportionally equated amongst all His creation in relation to His justice and judgements. He does not oppress a single one from among them the equivalent of even an atom’s weight. If there is a good deed, He multiplies it and gives from Himself a great reward. And every soul will be compensated for what it earned, and they will not be treated unjustly. Such is the meaning of His encompassing mercy in the ākhirah for all His creation, for which His Name al-Raḥmān is actualised. As for the mercy He has specified for the believers in this dunyā, for which His Name al-Raḥīm is denoted as He says: ‘

وَكَانَ بِالْمُؤْمِنِينَ رَحِيمًا ‎﴿٤٣﴾‏

And ever is He, to the believers, Merciful’
[Al-Aḥzāb, 33:43]

it is manifested in the tenderness He has shown specifically towards them relating to their dīn, compared to those He has abandoned from among those who have chosen disbelief in Him. As for what is prepared specifically in the ākhirah [from reward], He is al-Raḥīm to the believers in relation to it rather than the disbelievers. These are the aforementioned bounties and unfathomable favours we have alluded to that have been prepared specifically for none other than the believers.

Source: Tafsīr al-Ṭabarī 1:128-129
Translated by: Riyāḍ al-Kanadī