In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Allaah [The Most High] said:
فَٱعۡلَمۡ أَنَّهُ ۥ لَآ إِلَـٰهَ إِلَّا ٱللَّهُ وَٱسۡتَغۡفِرۡ لِذَنۢبِكَ وَلِلۡمُؤۡمِنِينَ وَٱلۡمُؤۡمِنَـٰتِۗ وَٱللَّهُ يَعۡلَمُ مُتَقَلَّبَكُمۡ
So know [O Muhammad] that Laa ilaaha ill-Allah [none has the right to be worshipped but Allah], and ask forgiveness for your sin, and also for [the sin of] believing men and believing women. And Allah knows well your moving about, and your place of rest [in your homes, graves etc]. [Surah Muhammad. Aayah 19]
It is Fard Ayn [i.e. an individual duty (or obligation)] to have knowledge of Laa Ilaaha Ilal laah [i.e. that none has the right to be worshipped except Allaah], because it is knowledge a person cannot do without. This knowledge can be acquired through several ways and the greatest of them is to seek knowledge about the sound meanings of Allaah’s Names and Attributes, because that teaches a person the Greatness of Allaah, directs him to deify and worship Allaah alone and makes him know that Allaah alone is worthy of all praise.
To know that Allaah [The Most High] alone is the Creator, the One in control of all affairs of the universe and this necessitates that He alone has the right to be deified and worshipped.
To know that Allaah [The Most High] alone bestowed the creation with all the apparent blessings they have [i.e. Islaam, the lawful pleasures of this world- including health, good looks, etc] and all the hidden blessings [i.e. Eemaan, knowledge, wisdom, guidance for doing righteous deeds, and also the pleasures and delights of the Hereafter in Paradise, etc]. All this necessitates that the hearts should be attached to Allaah, love Him, deify Him alone and associate no partners to Him.
To pay attention to what one sees and hears regarding the reward Allaah [The Most High] bestows on those who establish Laa Ilaaha Ilal laah- the help He gives them and the bliss He has kept for them in the afterlife; and the punishment He inflicts on the enemies of Tawheed- those who associate partners with Him in worship. All this shows that Allaah alone has the right to be worshipped.
To know the characteristics of the idols and the other false deities that are worshipped besides Allaah due to the fact that they are deficient in every way and dependent on others. Neither do they have the ability to harm nor benefit; neither do they have the ability to cause death nor give life, nor can they raise the dead. Neither can they aid those who worship them nor can they give them an atom’s weight of benefit, nor repel an atom’s weight of harm from them. The one who knows that these are some of the characteristics of the idols and false deities will know that none has the right worshipped except Allaah and that everything worshipped besides Allaah is false.
To know that the best of the creation-those people who possess the noblest and most perfect manners, the most sound intellects, views and the most perfect in knowledge- are the Messengers and Prophets of Allaah and then the pious learned scholars, and they all testify to the fact that Allaah alone has the right to be worshipped.
[Source: An Excerpt from ‘Tayseer Al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan). Slightly paraphrased]