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Surah 6. Al-An'am
Ayah 19 - 21
19قُلْ أَىُّ شَىْءٍ أَكْبَرُ شَهَـٰدَةً ۖ قُلِ ٱللَّهُ ۖ شَهِيدٌۢ بَيْنِى وَبَيْنَكُمْ ۚ وَأُوحِىَ إِلَىَّ هَـٰذَا ٱلْقُرْءَانُ لِأُنذِرَكُم بِهِۦ وَمَنۢ بَلَغَ ۚ أَئِنَّكُمْ لَتَشْهَدُونَ أَنَّ مَعَ ٱللَّهِ ءَالِهَةً أُخْرَىٰ ۚ قُل لَّآ أَشْهَدُ ۚ قُلْ إِنَّمَا هُوَ إِلَـٰهٌ وَٰحِدٌ وَإِنَّنِى بَرِىٓءٌ مِّمَّا تُشْرِكُونَ
20ٱلَّذِينَ ءَاتَيْنَـٰهُمُ ٱلْكِتَـٰبَ يَعْرِفُونَهُۥ كَمَا يَعْرِفُونَ أَبْنَآءَهُمُ ۘ ٱلَّذِينَ خَسِرُوٓا۟ أَنفُسَهُمْ فَهُمْ لَا يُؤْمِنُونَ
21وَمَنْ أَظْلَمُ مِمَّنِ ٱفْتَرَىٰ عَلَى ٱللَّهِ كَذِبًا أَوْ كَذَّبَ بِـَٔايَـٰتِهِۦٓ ۗ إِنَّهُۥ لَا يُفْلِحُ ٱلظَّـٰلِمُونَ
[19-21] Ask them, "Whose testimony is the most reliable?" Say "Allah is witness11 between me and you (that I am His Messenger), and this Qur'an has been revealed to me so that I should thereby warn you and all whom it may reach. What, will you really testify that there are other deities besides Allah?"12 Say, "As for me, I will never testify such a thing."13 Say, "Indeed, He alone is the Deity and I feel disgusted with the shirk you practice." Those, to whom We have given the Book, recognize this with as much certainty as they recognize their own sons.14 But those, who have made themselves liable to perdition, do not believe in this. And, who can be more unjust than the one who brings false allegations against Allah15 or denies Allah's Signs?16 Indeed, such unjust people can never attain to true success.
11That is. "Allah is a witness that I have been appointed a Messenger by Him and whatever I say is from Him."
12Mere guess-work or speculation does not suffice for testifying to anything: none can testify to a thing with certainty without the necessary knowledge about it. The question implies: "Do you really have the knowledge that there is any other Sovereign with authority than Allah, Who is worthy of service and worship?"
13That is, "You may, if you so like, give false testimony without any knowledge, but, as for me, I cannot give such a testimony."
14It means that those, who possess the knowledge of revealed Books, know it for certain that there is only One God, Who has no partner in His Godhead. Therefore they can discern the right creed about Allah from among many different kinds of other creeds' and theories about Godhead, just as any person can identify his own child from among many thousands of other children.
15"False allegation" is the claim that there are many other partners with Allah in His Godhead who have divine attributes and powers and are thus worthy of worship. It is also false to allege that Allah Himself has made such and such His special courtiers and has enjoined (or at least approved) that Divine attributes should be ascribed to them and that they should be treated with reverence, like Allah.
16"Allah's Signs" are all those signs that point to the fact that there is only One God in the universe and that all others are His servants. These signs are spread all over the universe. They are found in the person of man himself and in the character and the great achievements of the Prophets and in the revealed Books. That is why the one, who, in the presence of all these "Signs," ascribes Divine attributes to others and considers them worthy of Divine rights, is in fact guilty of gross iniquity. It is indeed a great injustice that one should ascribe such Divine attributes to others without any evidence, knowledge, observation and experience merely on the basis of guess-work or the traditions of the forefathers. As a result of this false creed, he does injustice to Truth, to Reality, to his own self and to everything and everyone he deals with in this universe.