This excerpt is taken from Sheikh Irshad Ullah Maan's book The Search for Truth.
Question: What did the Prophets (peace be upon them) invite their people to?
Answer: Allah Almighty says: [وَ لَقَدۡ بَعَثۡنَا فِیۡ کُلِّ اُمَّۃٍ رَّسُوۡلًا اَنِ اعۡبُدُوا اللّٰہَ وَ اجۡتَنِبُوا الطَّاغُوۡتَ] And indeed We sent messengers to every nation, [saying], "Worship Allah and avoid Taghut."
Reference: (An-Nahl:36)
It is understood that the first obligation upon the children of Adam is to disbelieve in Taghut and believe in Allah Almighty.
Question: What is the relation between believing in Allah Almighty and disbelieving in Taghut?
Answer: Belief in Allah is not possible until one disbelieves in Taghut. It is stated: [فَمَنۡ یَّکۡفُرۡ بِالطَّاغُوۡتِ وَ یُؤۡمِنۡۢ بِاللّٰہِ فَقَدِ اسۡتَمۡسَکَ بِالۡعُرۡوَۃِ الۡوُثۡقٰی لَا انۡفِصَامَ لَہَا] Whoever disbelieves in Taghut and believes in Allah has grasped the most trustworthy handhold which will never break.
Reference: (Al-Baqarah:256)
The Messenger of Allah ﷺ said: Whoever says ،،لا الہ الا اللہ،، and disbelieves in all those besides Allah who are worshipped, his wealth and blood are forbidden for the Muslims, and the matter of his heart is with Allah.
Reference: Muslim, Book of Faith, Chapter on the Command to Fight People... etc.: 23
Consider that merely reciting the Kalima with the tongue does not protect wealth and life; rather, the life and wealth of Muslims will be protected by the sword of Muslims only when they reject those deities whom the people of their time worshipped. This hadith provides clear evidence and explicit proof against those who only want to speak about monotheism. But today, those who recite the Kalima do not want to refute those whom they worship instead of Allah, because in their view, this does not create division in the Ummah.
Question: What is meant by Taghut?
Answer: Taghut is everything that is worshipped besides Allah and is pleased with being worshipped or causing others to worship it. (That is) Taghut refers to false claimants of divinity. The Prophets (peace be upon them) and the Awliya Allah called only to the worship of Allah and were the greatest enemies of those who worshipped others besides Allah; therefore, they are not Taghut, even if people worship them. There are three levels of a servant in comparison to Allah:
First level: This is that a person fundamentally considers obedience to Allah Almighty as the truth, but if he violates His commands, then this is immorality and he will be a sinner.
Second level: This is that he fundamentally deviates from His obedience and either becomes independent or starts serving someone else; this is polytheism and disbelief.
Third level: This is that he rebels against Allah and imposes his own rule over His creation. Whoever reaches this level is a tyrant (Taghut), and no person can be a believer until he rejects the tyrant.
Question: Who is the greatest tyrant (Taghut)?
Answer: Satan is the greatest tyrant who calls towards the worship of other than Allah. It is stated: [اَلَمۡ اَعۡہَدۡ اِلَیۡکُمۡ یٰبَنِیۡۤ اٰدَمَ اَنۡ لَّا تَعۡبُدُوا الشَّیۡطٰنَ ۚ اِنَّہٗ لَکُمۡ عَدُوٌّ مُّبِیۡنٌ] O Children of Adam! Did I not enjoin upon you that you should not worship Satan? Indeed, he is to you a clear enemy.
Reference: (Yusuf:60)
From this verse, it is also understood that obeying Satan is worshiping Satan. Therefore, unconditional obedience to other than Allah is His worship.