Allāh—the Most High—said:

فَاعْلَمْ أَنَّهُ لَا إِلَٰهَ إِلَّا اللَّهُ وَاسْتَغْفِرْ لِذَنبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ ۗ وَاللَّهُ يَعْلَمُ مُتَقَلَّبَكُمْ وَمَثْوَاكُمْ

“So know (O Muḥammad (صلى الله عليه وسلم)) that Lā ilāha ill-Allāh (none has the right to be worshipped but Allāh), and ask forgiveness for your sin, and also for (the sin of) believing men and believing women. And Allāh knows well your moving about, and your place of rest (in your homes).”
(Muḥammad, 47:19)

Imām ʿAbd al-Raḥmān ibn Nāṣir al-Saʿdī comments:

True knowledge must undoubtedly pertain to that which the heart finds satisfaction regarding, actively seeking knowledge of it. This is incomplete until one acts in accordance with his knowledge, as stipulated by each individual circumstance. The knowledge that is commanded in this verse is an individual obligatory duty upon every human being. It will never cease to apply to a single person, whosoever he may be.

The necessity of this knowledge and acting in accordance with it represents perfection in worshipping Allāh. It is, therefore, above all other necessities. Possessing knowledge of a matter is subject to knowing the paths towards the attainment of that knowledge, and subsequently traversing those paths. The path towards truly knowing the testification of “There is nothing worshipped in truth except for Allāh” may be summarised in the following matters:

Contemplating the Beautiful and Perfect Names and Attributes of Allāh

The greatest, strongest and most plainly evident reason is the contemplation of Allāh’s names, attributes, and actions which prove His perfection, greatness, and majesty. Indeed, true acknowledgement of them stipulates firmly knowing that none could ever be deserving of divinity besides Him. Just as it also stipulates the implementation of strife in recognition of that divinity and in worshipping Allāh in the most perfect fashion. For He is most deserving of praise, majesty, exaltation, whose beauty must be fully recognised.

Allāh Is the Creator and Originator of Creation

Knowing that He is alone in originating the creation, sustaining and providing for them, and controlling their affairs. This, then, denotes His oneness of divinity.
Allāh Is the Bestower of all Blessings
Knowing that all blessings—both apparent and unapparent, religious and worldly —are from Him alone. This must then cause one’s heart to be attached to Him, loving and seeking forgiveness from Him. Worshipping Him alone without ascribing partners to Him.

Allāh Is the Sole Granter of Rewards

The servant’s observation and acquaintance with the reward He has granted His close friends and allies. All of whom attested to His oneness. Among these rewards was the victory granted by Him to His prophets and their followers. Just as He has also expeditiously bestowed upon them apparent blessings, punishing His enemies who have ascribed partners to Him in worship. This is a clear evidence that proves He alone is deserving of all worship and divinity.

The Clear Deficiencies and Ungodly Attributes of the Idols and Intermediaries Worshipped Besides Allāh

Acknowledgement of the characteristics of the idols and that which has been set as equals that are worshipped besides Him, and taken as gods. Recognising their inherent state of destitution before Allāh from every conceivable angle and that they are deficient in every way. Completely devoid of the ability to benefit or harm themselves or their worshippers. Nor do they possess the ability to take life, nor grant it, nor to resurrect the dead. True acknowledgement of this must stipulate the complete falsehood of worshipping such things. That everything that is invoked besides Allāh is false, while Allāh is the only true God—the Manifest Truth.

The Agreement of All the Revealed Books

The complete corroboration of all the books of Allāh, their unanimous agreement on this testification.

The United Belief of all the Prophets and Messengers in Allāh’s Oneness

The complete agreement of all the prophets, messengers, and rabbānī scholars^[1] on it, their bearing witness to it. For they are the choicest class among this creation, the most complete in terms of their conduct, intellect, knowledge, and assuredness.

Contemplation of the Magnificence of Allāh’s Creation and His Perfect Ability to Sustain It

That which has been clarified and made to be plainly evident by Allāh from evidence that pertains to our own selves and the universe. All of which provides the greatest evidence for Allāh’s oneness, calling to belief in this testification both explicitly and implicitly. Due to these signs consisting of intricate, subtle design, amazing wisdom and engineering, and marvellous creations.

The Perfect Justice and Fairness of Allāh’s Legislation

The decisive laws that have been legislated by Allāh, all of which consist of uprightness, fulfilment of rights in equity and fairness, consisting of abundant goodness and righteousness. Just as they have been legislated in a way that instates that which is beneficial, completely avoiding that which represents harm, while bringing about a wide variety of goodness. This is among the greatest of evidences that prove that He is Allāh—there could be none deserving of being worshipped in truth save for Him. And that His legislation which has been revealed upon the tongues of His messengers all bear witness to this.

Thus, these paths whose varieties are innumerable have been clarified by Allāh in His book and are oft-repeated therein. The servants’ attention is brought to this purpose, which is among the most magnanimous and exalted of goals. Whosoever traverses a path among them will lead him to the assured belief that there is nothing worshipped in truth except for Allāh. With every additional path that a servant traverses from among them, desiring its traversal and increasing his knowledge of it, his assuredness regarding it becomes ever more abundant, his īmān firmer, more deeply rooted. To the extent that īmān in his heart becomes firmer than a mountain, sweeter to him than the most delicious of foods, more precious and valuable than one’s most valued asset.

Then the greatest and most inclusive of all these paths is deep contemplation of the glorious Qurʾān and reflection upon its verses. For it represents the greatest door towards belief in Allāh’s oneness. Acknowledgement of it occurs by means of general and detailed allusions to it in the Qurʾān which are not found elsewhere.

His saying: “and ask forgiveness for your sin”, that is: seek forgiveness from your Lord for your sin by incorporating the means by which forgiveness is achieved. This includes invoking Allāh for forgiveness and engaging in sincere repentance, deeds of righteousness that erase the perpetrated evil, abandonment of transgression, forgiving fellow created beings and being exemplary in one’s treatment of them. This is also inclusive of seeking forgiveness for them. For this reason, He said: “and also for (the sin of) believing men and believing women”. This represents the fruits of true īmān, that because of their faith, they acknowledge the right of every Muslim upon them, galvanising them towards seeking forgiveness on their behalf.

Also, if a servant has been ordered to seek forgiveness for every believing man and woman, this also denotes that one advises them with true sincerity. Loving for them the goodness he loves for himself, hating that evil should befall them just as he hates to be afflicted with it. He encourages them towards goodness and righteousness, forbids them from engaging in transgression and evil, all the while forgiving their faults and defects, overlooking their mistakes. He aspires towards unification and the fostering of amicability amongst their hearts, the complete removal of rancour and hatred that leads to enmity and division. For indeed, amicability lessens transgression while division is a means by which evil and disobedience is made abundant.

His saying: “And Allāh knows well your moving about, and your place of rest (in your homes)” that is, He knows your conduct and behaviour, your actions, your egress and ingress. He knows the outcome of your movements, and what you have settled upon. For He encompasses you, in any and all of your situations and states. In this statement there is both an awakening of desire, just as there is the instillment of fear from being recompensed for the actions one engages in, both righteous and evil.