[He] who made for you the earth a bed [spread out] and the sky a ceiling and sent down from the sky, rain and brought forth thereby fruits as provision for you. So do not attribute to Allah equals while you know [that there is nothing similar to Him].
Say, "Is it other than Allah I should desire as a lord while He is the Lord of all things? And every soul earns not [blame] except against itself, and no bearer of burdens will bear the burden of another. Then to your Lord is your return, and He will inform you concerning that over which you used to differ."
We note that even the way in which Allah unfolds His truths to us is uniquely ingenious:
When He addresses His repentants, They call them "believers" using His holy name: "Allah";
When He addresses His unrepentant, and so as not to impact them even by the designation, He includes them in "mankind" using "Lord".
"Lord" (Ar-Rab:الرَّبُ) comes from the verb to educate in Arabic. As a good education requires a rigor and a constructive firmness, Allah adds His mercy to reassure us so that we are receptive to His recommendations from which we are beneficiaries.
It is because He is "Lord of the worlds", that He is "The Entirely Merciful, The Especially Merciful".