Al-Jawāb as-Sahīh li-man baddala dīn al-Masīh - Ibn Taimiya - (6/30)

He has stated this in two passages of His Book. It is also the very first of the Ten Commandments that Allah revealed to Moses, when He said:

“I am Allah; there is no god but Me. I am your God who brought you out of the land of Egypt, out of bondage. You shall have no other god besides Me. Do not fashion an image or a likeness—of what is in the heavens above, or on the earth beneath, or in the waters under the earth. Do not bow down to them or worship them. Indeed, I am your Lord, the Almighty.”

The Messiah—peace be upon him—testified that this is the greatest commandment in the Law. Thus worshipping Allah alone with no partner and loving Him more than anything else is the supreme command and the loftiest word delivered by all the messengers—Moses, the Messiah, and Muhammad, may Allah’s blessings be upon them all.

The opposite of this is the shirk (polytheism) that Allah, exalted be He, does not forgive. He says:

“Yet among people are those who take rivals besides Allah, loving them as they should love Allah; but the believers are firmer in their love for Allah.” (al-Baqarah 2:165)

فِي مَوْضِعَيْنِ مِنْ كِتَابِهِ، وَهَذَا أَوَّلُ الْكَلِمَاتِ الْعَشْرِ الَّتِي أَنْزَلَهَا اللَّهُ عَلَى مُوسَى حَيْثُ قَالَ: أَنَا اللَّهُ لَا إِلَهَ إِلَّا أَنَا إِلَهُكَ الَّذِي أَخْرَجْتُكَ مِنْ أَرْضِ مِصْرَ، مِنَ التَّعَبُّدِ، لَا يَكُونُ لَكَ إِلَهٌ غَيْرِي ; لَا تَتَّخِذْ صُوَرًا، وَلَا تِمْثَالًا، مَا فِي السَّمَاوَاتِ مِنْ فَوْقُ، وَمَنْ فِي الْأَرْضِ مِنْ أَسْفَلُ، وَمَا فِي الْمَاءِ مِنْ تَحْتِ الْأَرْضِ ; لَا تَسْجُدْ لَهُنَّ ; وَلَا تَعْبُدْهُنَّ إِنِّي أَنَا رَبُّكَ الْعَزِيزُ. وَقَدْ شَهِدَ الْمَسِيحُ عَلَيْهِ السَّلَامُ أَنَّ هَذَا هُوَ أَعْظَمُ وَصِيَّةً فِي النَّامُوسِ، فَعِبَادَةُ اللَّهِ وَحْدَهُ لَا شَرِيكَ لَهُ، وَأَنْ يَكُونَ اللَّهُ أَحَبَّ إِلَى الْعَبْدِ مِنْ كُلِّ مَا سِوَاهُ، هُوَ أَعْظَمُ وَصِيَّةً وَكَلِمَةً جَاءَ بِهَا الْمُرْسَلُونَ كَمُوسَى، وَالْمَسِيحِ، وَمُحَمَّدٍ صَلَوَاتُ اللَّهِ عَلَيْهِمْ أَجْمَعِينَ، وَضِدُّ هَذَا هُوَ الشِّرْكُ الَّذِي لَا يَغْفِرُهُ اللَّهُ تَعَالَى: قَالَ تَعَالَى: {وَمِنَ النَّاسِ مَنْ يَتَّخِذُ مِنْ دُونِ اللَّهِ أَنْدَادًا يُحِبُّونَهُمْ كَحُبِّ اللَّهِ وَالَّذِينَ آمَنُوا أَشَدُّ حُبًّا لِلَّهِ} [البقرة: ١٦٥]