The unification of Ibaadah for Allaah Azzawajall alone and the avoidance of the Ibaadah of everything that is besides Him is that end for which the Messengers were sent, may the prayers and blessings of Allaah be upon them all. Allaah the Exalted said:

{And We have indeed sent to every Ummah (Nation) a messenger (with this command) that you should worship Allaah (alone) and avoid (the worship of) all false deities}[62]

Furthermore, this aim or objective (unifying all worship for Allaah alone) is the cause of the dispute of the messengers with their respective peoples and due to it mankind has become divided into two parties: a party in Paradise and a party in the Blazing Fire.

As for those who are in the Fire then they are those who deny Allaah Azzawajall absolutely; or acknowledge Allaah's Ruboobiyyah (Lordship) over the universe (meaning that He created, owns and regulates the heavens, the earth and all that is between them by Himself without any partners or helpers) but do not worship Him in His Uloohiyyah[63] by making their Ibaadah exclusively for Him alone or paying part worship to others besides Him. Such was the case of the pagans of Makkah to whom the Messenger (sallallaahu-alaihi-wasallam) was sent. The Exalted says:

{Say: 'Who provides for you from the sky and from the earth? Or who owns hearing and sight? And who brings out the living from the dead and brings out the dead from the living? And who disposes the affairs (of this universe)?' They will soon say: 'Allaah'. Say: 'Will you then not be afraid (of Allaah's punishment)?'}[64]

{Say: 'Who is the Lord of the Seven Heavens and the Lord of the Great Throne?' They will soon say: 'Allaah'. Say: 'Will you then not fear Him. Say: 'In whose Hand is the Sovereignty of everything and He protects (all) but against whom there is no protector if you really do have knowledge?'. They will soon say: 'Allaah'. Say: 'How then are you deceived and turn away from the Truth?'}[65]

So they believed in Allaah's Ruboobiyyah, even in some of His Names, but that did not make them Muslims. Rather, Allaah the Exalted called them disbelievers and made their abode the Hellfire, inspite of their affirmation of His Ruboobiyyah.

As for those who are in Paradise then they are those who have affirmed their belief in Allaah, the Exalted and everything that is due to Him from His Names and Attributes, who have believed in His Messengers and have responded to their call by making all their Ibaadah purely for Allaah alone; who travel upon their way, in both belief and action, and who then remain steadfast upon the Truth and strive for its dominance.

{Indeed those who say: "Our Rabb (Lord) is Allaah and thereafter Istaqaamoo (remain steadfast) [i.e. upon his sincere worship and all that it entails], on them shall be no fear, nor shall they grieve}[66]

So they acknowledge His Ruboobiyyah due to their saying "Our Lord is Allaah" and then they worship Him alone on the path of Istiqaama.[67]

Therefore making all of Ibaadah for Allaah Azzawajall alone is the basis of the Deen of Islaam as the meaning of Laa ilaaha illallaaha is "that there is nothing which has the right to Ibaadah in truth except Allaah alone". This meaning is indeed the straight and firm Deen which most of mankind are heedless of. The Exalted said:

{He has commanded that you do not worship except Him alone. That is the straight upright Deen yet most of mankind do not know}[68]