This question involves two issues, bid’ah (innovation) and shirk (polytheism, association of others with Allaah).
This issue may be divided into three topics:
1. Definition of bid’ah 2. Categories of bid’ah 3. Rulings on one who commits bid’ah – does that make him a kaafir or not?
1. Definition of bid’ah.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: “According to sharee’ah, the definition is ‘Worshipping Allaah in ways that Allaah has not prescribed.’ If you wish you may say, ‘Worshipping Allaah in ways that are not those of the Prophet (peace and blessings of Allaah be upon him) or his rightly guided successors (al-khulafaa’ al-raashidoon).’”
The first definition is taken from the aayah (interpretation of the meaning):
“Or have they partners with Allaah (false gods) who have instituted for them a religion which Allaah has not ordained?” [al-Shooraa 42:21]
The second definition is taken from the hadeeth of the Prophet (peace and blessings of Allaah be upon him), who said:
“I urge you to adhere to my way (Sunnah) and the way of the rightly-guided successors (al-khulafa’ al-raashidoon) who come after me. Hold fast to it and bite onto it with your eyeteeth [i.e., cling firmly to it], and beware of newly-invented matters.”
So everyone who worships Allaah in a manner that Allaah has not prescribed or in a manner that is not in accordance with the way of the Prophet (peace and blessings of Allaah be upon him) or his rightly-guided successors (al-khulafa’ al-raashidoon), is an innovator, whether that innovated worship has to do with the names and attributes of Allaah, or to do with His rulings and laws.
With regard to ordinary matters of habit and custom, these are not called bid’ah (innovation) in Islam, even though they may be described as such in linguistic terms. But they are not innovations in the religious sense, and these are not the things that the Prophet (peace and blessings of Allaah be upon him) was warning us against.
And there is no such thing in Islam as bid’ah hasanah (good innovation).”
(Majmoo’ Fataawa Ibn ‘Uthaymeen, vol. 2, p. 291)
2. Categories of bid’ah
Bid’ah may be divided into two categories:
(i) bid’ah which constitutes kufr
(ii) bid’ah which does not constitute kufr
If you ask, what is the definition of bid’ah which constitutes kufr and that which does not constitute kufr?
The answer is:
Shaykh Haafiz al-Hukami (may Allaah have mercy on him) said: “The kind of bid’ah which constitutes kufr is when one denies a matter on which there is scholarly consensus, which widely-known, and which no Muslim can have any excuse for not knowing, such as denying something that is obligatory, making something obligatory that is not obligatory, or making something haraam halaal, or making something halaal haraam; or believing some notion about Allaah, His Messenger and His Book when they are far above that, whether in terms of denial of affirmation – because that means disbelieving in the Qur’aan and in the message with which Allaah sent His Messenger (peace and blessings of Allaah be upon him).
Examples include the bid’ah of the Jahamiyyah, who denied the attributes of Allaah; or the notion that the Qur’aan was created; or the notion that some of the attributes of Allaah were created; or the bid’ah of the Qadariyyah who denied the knowledge and actions of Allaah; or the bid’ah of the Mujassimah who likened Allaah to His creation… etc.
The second category, bid’ah which does not constitute kufr, is defined as that which does not imply rejection of the Qur’aan or of anything with which Allaah sent His Messengers.
Examples include the Marwaani bid’ahs (which were denounced by the greatest Sahaabah who did not approve of them, although they did not denounce them as kaafirs or refuse to give them bay’ah because of that), such as delaying some of the prayers until the end of the due times, doing the Eid khutbah before the Eid prayer, delivering the khutbah whilst sitting down on Fridays, etc.
(Ma’aarij al-Qubool, 2/503-504)
3- The ruling on one who commits bid’ah – is he regarded as a kaafir or not?
The answer is that it depends.
If the bid’ah constitutes kufr, then the person is one of the following two types:
(i) Either it is known that his intention is to destroy the foundations of Islam and make the Muslims doubt it. Such a person is definitely a kaafir; indeed, he is a stranger to Islam and is one of the enemies of the faith.
(ii) Or he is deceived and confused; he cannot be denounced as a kaafir until proof is established against him, fair and square.
If the bid’ah does not constitute kufr, then he should not be denounced as a kaafir. Rather, he remains a Muslim, but he has done a gravely evil action.
If you ask, how should we deal with those who commit bid’ah?
The answer is:
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said: “In both cases, we have to call these people – who claim to be Muslim but who commit acts of bid’ah which may constitute kufr or may be less than that – to the truth, by explaining the truth without being hostile or condemning what they are doing. But once we know that they are too arrogant to accept the truth – for Allaah says (interpretation of the meaning), ‘And insult not those whom they (disbelievers) worship besides Allaah, lest they insult Allaah wrongfully without knowledge.’ [al-An’aam 6:108] – if we find out that they are stubborn and arrogant, then we should point out their falsehood, because then pointing out their falsehood becomes an obligation upon us.
With regard to boycotting them, that depends upon the bid’ah. If it is a bid’ah which constitutes kufr, then it is obligatory to boycott the person who does it. If it is of a lesser degree than that, then it is essential to examine the situation further. If something may be achieved by boycotting the person, then we do it; if no purpose will be served by it, or if it will only make him more disobedient and arrogant, then we should avoid doing that, because whatever serves no purpose, it is better not to do it. And also in principle it is haraam to boycott a believer, because the Prophet (peace and blessings of Allaah be upon him) said: ‘It is not permissible for a man to forsake [not speak to] his brother for more than three [days].’”
(Adapted from Majmoo’ Fataawa Ibn ‘Uthaymeen, vol. 2, p. 293)
B. Shirk, its types and the definition of each
Shaykh Muhammad ibn ‘Uthaymeen said:
“Shirk is of two types, major shirk which puts a person beyond the pale of Islam, and lesser shirk.”
The first type, major shirk, is “Every type of shirk which the Lawgiver described as such and which puts a person beyond the pale of his religion” – such as devoting any kind of act of worship which should be for Allaah to someone other than Allaah, such as praying to anyone other than Allaah, fasting for anyone other than Allaah or offering a sacrifice to anyone other than Allaah. It is also a form of major shirk to offer supplication (du’aa’) to anyone other than Allaah, such as calling upon the occupant of a grave or calling upon one who is absent to help one in some way in which no one is able to help except Allaah.
The second type is minor shirk, which means every kind of speech or action that Islam describes as shirk, but it does not put a person beyond the pale of Islam – such as swearing an oath by something other than Allaah, because the Prophet (peace and blessings of Allaah be upon him) said that whoever swears an oath by something other than Allaah is guilty of kufr or shirk.”
The one who swears an oath by something other than Allaah but does not believe that anyone other than Allaah has the same greatness as Allah, is a mushrik who is guilty of lesser shirk, regardless of whether the one by whom he swore is venerated by people or not. It is not permissible to swear by the Prophet (peace and blessings of Allaah be upon him), or by the president, or by the Ka’bah, or by Jibreel, because this is shirk, but it is minor shirk which does not put a person beyond the pale of Islam.
Another type of minor shirk is showing off, which means that a person does something so that people will see it, not for the sake of Allaah.
The ways in which showing off may cancel out acts of worship are either of the following:
The first is when it is applies to an act of worship from the outset, i.e., the person is not doing that action for any reason other than showing off. In this case, the action is invalid and is rejected, because of the hadeeth of Abu Hurayrah which was attributed to the Prophet (peace and blessings of Allaah be upon him), which says that Allaah said, “I am so self-sufficient that I am in no need of having an associate. Thus he who does an action for someone else’s sake as well as Mine will have that action renounced by Me to him whom he associated with Me.”
(Narrated by Muslim, Kitaab al-Zuhd, no. 2985)
The second is when the showing off happens later on during the act of worship, i.e., the action is originally for Allaah, then showing off creeps into it. This may be one of two cases:
The first is when the person resists it – this does not harm him.
For example, a man has prayed a rak’ah, then some people come along during his second rak’ah and it occurs to him to make the rukoo’ or sujood longer, or makes himself weep, and so on. If he resists that, it does not harm him, because he is striving against this idea. But if he goes along with that, then every action which stemmed from showing off is invalid, such as if he made his standing or prostration long, or he made himself weep – all of those actions will be cancelled out. But does this invalidation extend to the entire act of worship or not?
We say that either of the following must apply:
Either the end of his act of worship was connected to the beginning (with no pause); so if the end of it is invalidated then all of it is invalidated.
This is the case with the prayer – the last part of it cannot be invalidated without the first part also being invalidated, so the whole prayer is invalid.
Or if the beginning of the action is separate from the end of it, then the first part is valid but the latter part is not. Whatever came before the showing off is valid, and what came after it is not valid.
An example of that is a man who has a hundred riyals, and gives fifty of them in charity for the sake of Allaah with a sound intention, then he gives fifty in charity for the purpose of showing off. The first fifty are accepted, and the second fifty are not accepted, because the latter is separate from the former.”
Majmoo’ Fataawa wa Rasaa’il Ibn ‘Uthaymeen, and al-Qawl al-Mufeed Sharh Kitaab al-Tawheed, vol. 1, p. 114, 1st edition