Riba is forbidden according to the Qur’aan and Sunnah, and definitive scholarly consensus. Whoever regards it as permissible is a kaafir, because the basic principle is that whoever rejects something on which there is obvious scholarly consensus is guilty of kufr.
Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) said:
The belief that the well-known obligations are obligatory and that the well-known prohibitions are forbidden is one of the most important basic principles of faith, and the one who rejects that is a kaafir according to scholarly consensus.
Majmoo’ al-Fataawa, 12/497.
Ibn Qudaamah (may Allaah have mercy on him) said:
Whoever believes that something is permissible when there is consensus that it is forbidden and the ruling thereon is well known among the Muslims and the texts leave no room for doubt concerning it – such as pig meat, adultery and the like, concerning which there is no difference of scholarly opinion – is guilty of kufr.
Al-Nawawi (may Allaah have mercy on him) said:
But nowadays Islam has spread and the knowledge that zakaah is obligatory is well known to the elite and commoners alike; the scholar and the ignorant are both aware of it. So no one has any excuse for misinterpreting (the texts) and denying it. The same applies to everyone who denies any matter of religion on which the ummah is agreed, if knowledge of it is widespread, such as the five daily prayers, fasting the month of Ramadaan, doing ghusl to cleanse oneself of janaabah (impurity following sexual activity), the prohibition on adultery, alcohol and marrying mahrams, and other rulings, unless he is new in Islam and does not know its rulings, in which case if he denies something out of ignorance he is not guilty of kufr… But with regard to matters on which there is consensus but which may only be known by those with specialized knowledge, such as the prohibition on being married to a woman and her paternal aunt or maternal aunt at the same time, or that the one who kills with intent cannot inherit (from his victim), or that the grandmother is entitled to one-sixth (of the estate), and other such rulings, then whoever denies them is not guilty of kufr, rather he is to be excused because knowledge of these things is not widespread among the common folk.
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: The ruling on riba is that it is haraam according to the Qur’aan and Sunnah and scholarly consensus. It is classified as one of the major sins, because Allaah says (interpretation of the meaning):
“but whoever returns (to Ribaa), such are the dwellers of the Fire — they will abide therein”
“And if you do not do it, then take a notice of war from Allaah and His Messenger”
and because the Messenger of Allaah (peace and blessings of Allaah be upon him) cursed the one who consumes riba, the one who pays it, the two who witness it and the one who writes it down. So it is a major sin.
There is scholarly consensus that it is forbidden, hence whoever denies that it is forbidden although he lives in a Muslim environment is an apostate, because this is one of the obviously forbidden things on which there is consensus.
But if we say this, does that means that the scholars are agreed on all forms of riba? The answer is no; there is a difference of opinion concerning some forms of it. This is like what we have said about zakaah being obligatory according to consensus, but despite that there is no consensus on every form of it. They differed concerning the zakaah on camels and oxen that are used for farming or irrigation, and they differed concerning the zakaah on jewellery and the like, but in general the scholars are agreed that riba is haraam and is a major sin.
Al-Sharh al-Mumti’ ‘ala Zaad al-Mustanqi’, 8/387
Based on that, it may be said:
Whoever denies that riba is forbidden is a kaafir, because the fact that it is forbidden is one of the matters indicated by the texts, and the scholars are obviously agreed that it is haraam, and this is well known among the Muslims.
But if he denies that a specific form of riba is forbidden, concerning which there is a difference of opinion among the scholars or there is no consensus that it is obviously forbidden, then he is not a kaafir; rather his case should be examined further. He may be a mujtahid who will be rewarded for his ijtihad, or he may be excused, or he may be an evildoer if by regarding this as permissible he is following his own whims and desires.
And Allaah knows best. May Allaah send blessings and peace upon our Prophet Muhammad.