In the name of allah, the Beneficent, the Merciful What is sin in Islam?

is a Muslim,

one for whom there is guarantee (of the safety of his life and property) from Allah and

the guarantee from His Messenger.

Therefore, break not the covenant of Allah with regard to his protection.”

(Bukhari)

In this Hadith Islamic slaughtering has been described as a sign of one's being a Muslim like the prayer and facing the Qibla. In another Hadith the Messenger of Allah said about the Magians that they should be dealt with like People of the Book, except in two matters i.e. it is not lawful for a Muslim to marry with a Magian woman and eat their slaughter.
It is evident from this Hadith that although marriage is a social necessity and among the natural needs of human beings, yet Islam has laid down certain restrictions on it which are necessary to be fulfilled, otherwise marriage will not be legal under the Shari'ah. It is the case with slaughter on which certain restrictions imposed by the Shari'ah are well known to Muslims of every age and sect. It is regarded as one of the fundamentals of Deen, so there is no need to give proof in support of the accepted principles.
Three words have been used in the Holy Qur'an for slaughtering according to the Islamic way i.e. Zakah, Zibaha and Nahar. Zakah is a common word used for Zibah and Naharas, well. It is unanimously regarded a Qur'anic term just like Salat, and Saum. We regard only that meaning of salat and saum as authentic, which is proved by other verses of the Holy Qur'an and the teachings of the Prophet. It it a distortion to make an inference merely from its literal meaning. Zakah is a purely Islamic technical term having its two forms i.e. optional and non-optional, which have been mentioned in the Holy Qur'an. Separate regulations have been prescribed for both. Narrators of Ahadith and Jurists have called the optional Zakah as Zibah and the non-optional as shayd (game) respectively. But there are certain requirements and conditions as laid down in the Qur'an and Sunnah, which have been explained earlier.
But Mufti Abduh by overriding the verdicts of all the jurists and commentators of the Qur'an attributed a new meaning to the said Qur'anic term. The gist of his research is that for Zakah it will be sufficient to kill an animal with the intention of eating, whatever may be form of killing. He equated non-optional Zakah against clear elaborations found in the Qur'an and the Sunnah with optional Zakah by his own guesswork. He, therefore, refused to accept the basic condition of cutting veins of throat in intentional slaughtering which is a prescribed condition according to the unanimous opinion of the Ummah. He even went further and said that killing of an animal by electric shock (stunning) is also lawful (Halal). It is not only Halal but is also a better and preferable method. The whole discussion and detail is given in his famous commentary of the Holy Qur'an called Tafseer AI-Manar (vol. 6 P. 144). One sentence of the commentary being

"And I believe that if the Prophet of Allah (peace be upon him) had knowledge of any method of slaughtering which may facilitate and prove painless for the animal, such as killing by electric shock (stunning), the Prophet of Allah would certainly have declared it superior to the Islamic method of slaughtering."
Mufti Abduh declared that killing by electric shock (stunning) was not different from slaughtering. It was really a great audacity on the part of Mufti Abduh that by insisting on his baseless and wrong concept, he claimed that if the Prophet of Allah (saw) had knowledge of this method, .he would have certainly given up the Islamic Way and adopted it. (A matter of great sorrow indeed).
A complement of his aforesaid Ijtehad is that if we kill an animal by strangulation, it is also Halal. He has tried to answer this open violation of the Qur'anic verse by taking support of a wrong discussion over “strangulated” which is against the consensus of the companions of the holy Prophet, Tabeen and the majority of the Ummah. (AI-Manar, vol. VI p. 137).
Mufti Abduh had already denied the necessity of reciting Allah's name at the time of slaughter, now he did away with the requirement of cutting throat's veins. Animals killed intentionally by strangulation also become Halal. According to his investigation the only Haram animal is that which dies a natural death or without the interference of any man dies by falling from any high place or by automatic strangulation. Any animal which is killed with the intention that it will be used by human beings is Halal: whoever may be the killer, whatever may be the method of killing, whether he recites Allah's name or not, the slaughterer may be a Muslim or a non-Muslim, whether he cuts the veins of throat or not, all is lawful according to Mufti Abduh. His research particularly about the people of the Book is that the food of the people of the Book is Halal without any condition or restriction whether they have killed the animal by strangulation or electric tock or by any other method. (AI-Manar vol. VI, p. 200).
He was kind enough towards Muslims in so far as he did not declare the swine flesh Halal for them which is also included in the food of People of Book, though the basic purpose of his commentary was to prove in general that the food of the " People of the Book " is lawful for Muslims. Consequently the meat of pig as well could have entered the list.
Proceeding ahead he openly said that eating of animals meat is one of the natural habits of human beings having no link with religion. Religious restrictions are confined only to acts of worship. His words are as under :-
“The customary matters relating to eating and clothing are not from laws of rituals ordained by Allah for the people to follow. The laws relating to rituals are established by the text (nass) of the law-Giver."
In a nutshell Mufti Abduh's Ijtehad is nothing except that the distinction of Halal and Haram in the articles of eating, drinking, dressing and of daily use, is meaningless. If such type of Ijtehad is correct, then marriages and divorces are also amongst the matters of habit and custom. Hence discussion of Halal and Haram about the same is useless and religious restrictions imposed on them are wrong.
There can't be a more suitable ljtehid in the modern age of absolute freedom and religious indifference than that of Mufti Abduh. As such the Westernized generation liked it very much.
It was an error, no doubt a great error of Mufti Abduh and Allama Rashid Reza, yet in the light of their services in the field of education and with the grace of Allah, it is hoped that Allah might forgive them for giving a wrong verdict. We are, however, worried about the people who under some pretext and without any solid argument have followed his opinion only for case and comfort and the satisfaction of their baser selves.
The commission of an error is possible, howsoever great a scholar may be. It is a well-known saying in Arabic: