There are many people today who admit to Allah being their Lord. They understand Allah created them and the beautiful world they reside in, and that they are provided sustenance by His Mercy and Will. You will find them also wholeheartedly agreeing to the perfect attributes of Allah as being the Creator, Rubb, The Most Merciful, The Sustainer. Yet, even after believing they commit the injustice of shirk against their Lord, a situation very similar to the disgraced Arabs of Jahiliyyah during the time of the Prophet Muhammad (pbuh): they believed in Allah as their Lord, yet they strove to mock and fight the purely monotheistic message of Islam.

Say, "Who provides for you from the heaven and the earth? Or who controls hearing and sight and who brings the living out of the dead and brings the dead out of the living and who arranges [every] matter?" They will say, " Allah ," so say, "Then will you not fear Him?" [Quran 10:31]

Allah relates above the acknowledgment by the Arabs of Jahiliyah of His unique position as the one and only God, the One in full control. But despite their belief, Allah gave them a severe reprimand because they had a major defect: they did not worship Allah alone. To understand the injustice involved imagine working for someone where you receive a regular paycheck and generous bonuses from time to time, but instead of working for your employer you go and support a competitor. Would this be a fair or unjust dealing? Allah had bestowed numerous favors upon them, their livelihoods, their families, and the opportunity to live next to the sacred Kaabah, yet they engaged in asking, praising and worshiping others beings for their bounty.

But when He gives them a good [child], they ascribe partners to Him concerning that which He has given them. Exalted is Allah above what they associate with Him. Do they associate with Him those who create nothing and they are [themselves] created? [Quran 7:190-191]

Allah constantly iterates in the Quran that He has no partners, and anyone who worships other than Him has essentially raised His co-equal. When you consider that any object taken for worship is another creation of Allah, with no ability of its own except what Allah allows, it begs the question why would one spend time and resources in such an act. Allah is the Creator, and the only One deserving of all types of worship.

O people, an example is presented, so listen to it. Indeed, those you invoke besides Allah will never create [as much as] a fly, even if they gathered together for that purpose. And if the fly should steal away from them a [tiny] thing, they could not recover it from him. Weak are the pursuer and pursued. [Quran 22:73]