Islam is the Religion of Abraham (Millatu Ibrahim) – The Polytheist Contradicts This Fundamentally
As has already become clear from the above verses, Islam is the religion of Abraham, which is repeatedly mentioned in the Qur'an with the term millatu Ibrahim and explained in various ways.
The religion of Ibrahim means worshiping only Allah. It is the universal Islam with which all prophets عليهم السلام were sent. Therefore, it is self-evident that the Prophet of Islam and likewise his followers were commanded to adhere to this religion. Nevertheless, this matter is explicitly mentioned and affirmed in the Qur'an. Regarding this, it is stated in the Qur'an:
﴿ثُمَّ أَوْحَيْنَا إِلَيْكَ أَنِ اتَّبِعْ مِلَّةَ إِبْرَاهِيمَ حَنِيفًا وَمَا كَانَ مِنَ الْمُشْرِكِينَ﴾
Then We revealed to you, [O Muhammad], to follow the din of Abraham, as a hanif; and he was not of the mushrikin.
[sura an-Nahl, 16:123]
This also demonstrates that a polytheist cannot have fulfilled the religion of Ibrahim and, consequently, Islamic monotheism. As mentioned, this is evident in numerous places in the Qur'an. However, what has been stated so far shall suffice for us in light of the scope of this concise book.
If Islam Were Merely a Verbal Declaration, the Arabian Idolaters Would Have Been Muslims
If it were possible for a person to become a follower of Islam merely through verbal declaration while fundamentally contradicting this declaration through various actions, statements, and beliefs, then all pre-Islamic idolaters would undoubtedly have to be considered actual followers of the religion of Ibrahim.
There is no doubt that those idolaters themselves claimed to belong to the religion of Ibrahim. However, many of them were completely unaware that their idolatry contradicted the very foundation of that religion. They were ignorant of it.
Despite this ignorance, there has always been a consensus among Muslims not to refer to those idolaters as Muslims but rather as mushrikun. None of the early scholars would have considered designating those idolaters as Muslims, hunafa', or monotheists merely because of their verbal declaration. This matter is so clear that, to this day, hardly anyone makes such a claim.
A person who now associates himself with the Islam of Muhammad صلى الله عليه وسلم but contradicts its foundation just as the pre-Islamic idolaters did can, just as clearly, not be considered a monotheist. In this regard, it is entirely irrelevant whether the individual is aware of his own condition or not, because a person's awareness of his situation does not change the fundamental question of whether he is, in fact, worshiping someone other than Allah – thus committing polytheism – or worshiping only Allah and thereby realizing monotheism.