the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
Juristic consensus (ijmâ’) is the agreement of all of the people of knowledge of a given generation (whether it is the generation of the Companions or any generation thereafter) on a matter of Islamic Law.
When this consensus occurs, it indicates to us that there can be no doubt about the ruling that was arrived at, since the Prophet (peace be upon him) has informed us that his followers would never agree on a falsehood.
Rejecting a matter wherein there is consensus is always an error, but it is not always tantamount to unbelief. There are different types of rulings upon which there is ijmâ`. Some of those are the matters that are known by all Muslims necessarily (mâ `ulima min al-dîn bil-darûrah). These include the essentials of faith, the five pillars, that the fard prayers are five, that we must fast the month of Ramadan, that fornication and murder are unlawful, …etc. A person who denies these things is outside the fold of Islam.
However, even then the proof must be established against the person and all factors that prevent a ruling of unbelief – like newness to Islam, excusable ignorance due to remoteness …etc – need to be ruled out. Only rejection of these matters that are known by all Muslims necessarily (mâ `ulima min al-dîn bil-darûrah) can bring a person outside of Ahl al-Sunnah wa al-Jamâ`ah and can possibly lead a person to unbelief.
As for other, less well-known legal matters, the consequences are less serious. First of all, it has to be ascertained that the scholar is aware that consensus exists on the matter. It also has to be a matter about which consensus is established with certainty, since most claims of consensus are contentious or at least open to question.
If a scholar goes against an indisputably established point of consensus in such lesser-known matters, then he is mistaken, no more. It will not be lawful for others to follow his ruling.
If he admits to the existence of consensus in one of those less well-known legal matters and acknowledges it but still decides to disregard it, he will be sinning, but that will be the extent of it.
And Allah knows best.
Source: Islam Today