Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
Ar-Rahman and Ar-Rahim are two of the Names of Allah تعالى. Both indicate that He سبحانه وتعالى possesses mercy, but there is a difference in their indication: Ar-Rahman indicates the vastness and greatness of mercy, while Ar-Rahim indicates that this mercy reaches His creation.
Ar-Rahman means the One whose mercy is vast and immense. This Name indicates the vastness and fullness of the mercy of Allah تعالى.
Shaykh Ibn ‘Uthaymeen رحمه الله said:
“Ar-Rahman is the One whose mercy is vast, because the pattern fa‘lan in the Arabic language indicates vastness and fullness, just as one says: ‘an angry man’ when he is filled with anger.”
Thus, the Name Ar-Rahman indicates that the mercy of Allah سبحانه وتعالى is vast and immense, beyond the ability of creation to fully comprehend.
As for Ar-Rahim, it is a Name that indicates action, because it follows the pattern fa‘il, meaning one who performs an action. Thus, it indicates the granting and conveying of mercy to whomever Allah تعالى wills among His creation.
Ibn ‘Uthaymeen رحمه الله said:
“Ar-Rahim is a Name that indicates action, because it is on the pattern fa‘il, meaning one who performs an action; therefore, it indicates action.”
Accordingly, the Name Ar-Rahim indicates that the mercy of Allah تعالى reaches His servants, and that He has mercy upon them and bestows His favor upon them through His mercy.
Based on this, the two Names combine two great meanings:
Ar-Rahman: indicates the vastness and greatness of the mercy of Allah تعالى.
Ar-Rahim: indicates the reaching of Allah تعالى’s mercy to His creation.
For this reason, some scholars have said that Ar-Rahman indicates the general mercy, while Ar-Rahim indicates the special mercy bestowed upon the believers.
However, Shaykh Ibn ‘Uthaymeen رحمه الله preferred describing the distinction between the two Names in terms of the vastness of mercy and its reaching the creation, as this combines the indications of both Names in a clearer manner.
Thus, when it is said: “Ar-Rahman, Ar-Rahim,” this indicates that the mercy of Allah تعالى is vast and immense, and reaches His servants.
Ar-Rahman and Ar-Rahim are both among the Beautiful Names of Allah, and both indicate that mercy is established as an attribute of Allah تعالى. The difference between them is that “Ar-Rahman” indicates the vastness and greatness of mercy, while “Ar-Rahim” indicates that mercy reaches His creation.
And Allah knows best.