To truly understand Shirk (الشِّرْك), one must first understand Tawḥīd (التوحيد).
Tawḥīd is to affirm the Kalimah Ṭayyibah (الكلمة الطيبة):
لَا إِلٰهَ إِلَّا اللَّهُ
Lā ilāha illā Allāh
"There is none worthy of worship except Allah."
The foundation of this testimony begins with لَا (Lā)—"No." It is a complete rejection of every false deity and every object of worship besides Allah.
The opposite of لَا (Lā) is نَعَمْ (Naʿam)—"Yes."
Therefore, the essence of Shirk is as though one were to say:
نَعَمْ، إِلٰهٌ غَيْرُ اللَّهِ or "نَعَمْ، إِلٰهٌ إِلَّا اللَّهُ"
"Yes, there are deities worthy of worship besides Allah."
This is not an actual Islamic formula, but a conceptual illustration of the opposite of the Kalimah. Shirk is to affirm or accept that anyone or anything besides Allah deserves worship.
Thus, Tawḥīd begins with "Lā" (لا)—the rejection of all false gods—and ends with "illā Allāh" (إِلَّا اللَّهُ)—the affirmation that Allah alone is worthy of worship.
One example from the Holy Qur'an:
لَقَدْ كَفَرَ الَّذِينَ قَالُوا إِنَّ اللَّهَ هُوَ الْمَسِيحُ ابْنُ مَرْيَمَ
"They have certainly disbelieved who say, 'Allah is the Messiah, the son of Mary.'" (Qur'an 5:72)
Notice the phrase:
اللَّهُ هُوَ الْمَسِيحُ
"Allah is the Messiah."
Now, replace "the Messiah" with any other being—whether Muḥammad ﷺ (مُحَمَّدٌ ﷺ), Jibrīl (جِبْرِيلُ), or any righteous servant of Allah. In Islam, we never equate anyone with Allah or attribute divinity to anyone besides Him.
Instead, we say Rasūlullāh (رَسُولُ اللَّهِ), ʿAbdullāh (عَبْدُ اللَّهِ), and Awliyāʾullāh (أَوْلِيَاءُ اللَّهِ). Even when we hear of a great miracle (muʿjizah – مُعْجِزَة) or a karāmah (كَرَامَة), a Muslim says Allāhu Akbar (اللَّهُ أَكْبَرُ), never "Rasūl Akbar."
In this way, Islam firmly safeguards the believer from falling into major Shirk.
"As a very basic principle, to take someone else or equate anyone as Ilāh (deity worthy of worship) is the essence of Shirk."
Those who accuse Muslims of Shirk often do not apply the definition of Shirk as explained by the traditional scholars of Islam, or they have not fully understood the concept of Tawḥīd.
وَاللَّهُ أَعْلَمُ
Allah knows best.