We refer to Prophet Abraham and his devotion to God alone
The Islam is
to follow Prophet Abraham who was before the revealing of the Torah and the Gospel.
He was a monotheist: devoted to God alone, and was against the idolatry, and in fact he broke up the idols of his people.
The thing, that makes people fall into the idolatry and associating others with God, is the
enthusiasm about their prophets and religious leaders, to the extent that they ascribe to their apostles the attributes of God Almighty.
>>> In other words: people will start to
exaggerate in loving their apostle, until they
glorify him as do they glorify God Most Gracious, then they
worship him together with the Creator of the universe.
This is in the Quran 4: 171, forbidding such enthusiasm:
يَا أَهْلَ الْكِتَابِ لاَ تَغْلُواْ فِي دِينِكُمْ وَلاَ تَقُولُواْ عَلَى اللّهِ إِلاَّ الْحَقِّ إِنَّمَا الْمَسِيحُ عِيسَى ابْنُ مَرْيَمَ رَسُولُ اللّهِ وَكَلِمَتُهُ أَلْقَاهَا إِلَى مَرْيَمَ وَرُوحٌ مِّنْهُ فَآمِنُواْ بِاللّهِ وَرُسُلِهِ وَلاَ تَقُولُواْ ثَلاَثَةٌ انتَهُواْ خَيْرًا لَّكُمْ إِنَّمَا اللّهُ إِلَهٌ وَاحِدٌ سُبْحَانَهُ أَن يَكُونَ لَهُ وَلَدٌ لَّهُ مَا فِي السَّمَاوَات وَمَا فِي الأَرْضِ وَكَفَى بِاللّهِ وَكِيلاً
The explanation:
(O people of the Bible [lit. Scripture],
do not exaggerate in your religion nor say aught concerning God save the truth.
The Messiah Jesus the son of Mary was but a messenger of God [like many other apostles that had passed before him], and His word, that He cast to Mary and a spirit [coming] from His [neighborhood],
so believe in God and [all] His messengers.
Say not "Trinity"; desist [from such words] it will be better for you; for God is One God;
Far Exalted is He above having any son.
To Him belong all things in the heavens and on earth.
God suffices for a One Who registers their works.)
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And this is in the Gospel according to Mark 12: 32 “
There is One God and there is no other besides Him.”
http://quranandhebrewbible.t35.com