Four Basic Terms
Ilah, Rabb, Deen and 'Ibadah, are four terms basic to the whole teaching of the Qur'an.
Through- out its pages, it stresses again and again that Allah Almighty is the Rabb and
the Ilah ;that there is no ilah but He, nor is there any other rabb, nor does He share
with anyone else the qualities and attributes implied by these terms. He, and He alone
should therefore be accepted as one's Ilah and Rabb, and no-one else should in the
least be believed to possess the attributes which these words imply. It also demands
that we should give our ibadah to Him and Him alone, and not to anyone else, and
make our deen exclusive to Him and reject all other deens!
And to every Messenger We ordained before you (O Prophet), the message which We
gave (for himself and for others) was none other than that "There is no Ilah but Myself,
and therefore give your 'ibadah to Me alone. " (Quran 21:25)
...., And (they) were not Ordered except to give their 'ibadah to one Ilah, only; there is
no ilah except He, (end) He is free of (the taint of) what they attribute to Him by their
shirk (that is, by associating others with Him in His exclusive qualities and attributes).
Abul Ala Maududi http://en.wikipedia.org/wiki/Sayyid_Abul_Ala_Maududi