Ar-Rehman & Ar-Rahim

In the beginning of the Revelation days on the Prophet (SAW), among the most striking features were the 2 Divine names ar-Rahman and ar-Rahim. The word rahman an intensive form of rahim, merciful, was current in the sense of very merciful or boundlessly merciful. The still more intensive rahman, for lack of any concept of it, had fallen into disuse. The Revelation revived it in accordance with the new religion's basic need to dwell on the heights of Transcendence. Being stronger even than ar-Rahim (the All Merciful) the Name ar-Rahman refers to the very essence or root of Mercy , that is to the Infinite Beneficence or Goodness of God, and the Koran expressly makes it an equivalent of Allah: Invoke God (Allah) or invoke the Infinitely Good( ar-Rahman), whichever you invoke, His are the names most Beautiful."

From: Muhammad Umair Sami