Praise be to Allah, and peace and blessings be upon the Messenger of Allah, his family, and his Companions. To proceed:
The position of Ahl al-Sunnah wa’l-Jama‘ah is that a person who commits a major sin does not become a disbeliever merely by committing that sin, as long as he does not regard it as permissible or deny Allah’s ruling concerning it. Rather, he is a sinful Muslim whose faith is deficient, and he remains under Allah’s will: if Allah wills, He may punish him, and if He wills, He may forgive him.
As for the verse concerning riba (usury):
“Whoever returns to it—those are the companions of the Fire; they will abide therein.”
It refers to one who returns to riba while regarding it as permissible, denying its prohibition, or saying as the people of the pre-Islamic period said:
“Trade is just like riba.”
This is disbelief because it constitutes a rejection of Allah’s ruling and an objection to His law.
As for a Muslim who knows that riba is forbidden but nevertheless engages in it while acknowledging its prohibition, he is a sinner who has committed a grave major sin, but he is not judged to be a disbeliever merely because of that.
If the verse is understood as also applying to a sinful Muslim, then “abiding therein” is understood as remaining for a long period, not eternal and everlasting punishment, because the religious texts indicate that a person who dies upon Tawhid will not remain in Hell forever.
Among these texts is the saying of the Prophet ﷺ:
“Whoever says, ‘There is no god worthy of worship except Allah,’ and then dies upon that, will enter Paradise, even if he committed adultery and even if he stole.”
As for the verse in Surah As-Sajdah:
“As for those who defiantly disobeyed, their refuge will be the Fire.”
The context indicates that the “defiant disobedience” mentioned here refers to disbelief, because the preceding verses divide people into believers and defiantly disobedient people, and then say regarding the latter:
“Taste the punishment of the Fire which you used to deny.”
Denying the punishment of the Fire is a characteristic of disbelievers, not of a sinful Muslim who believes in Allah and the Last Day.
The term “fisq” (defiant disobedience) in the Quran and Sunnah is used in two ways:
Major fisq: disbelief that takes a person outside Islam.
Minor fisq: sinful disobedience that does not take a person outside Islam.
Examples include slander, drinking alcohol, adultery, and other major sins.
Conclusion
Ahl al-Sunnah do not declare someone who commits a major sin to be a disbeliever as long as he does not regard the sin as permissible.
The verse concerning riba applies to one who regards riba as permissible or rejects Allah’s ruling.
A Muslim who consumes riba while believing it to be forbidden is sinful, not a disbeliever.
The term “abiding” may refer to a prolonged stay rather than eternal and everlasting punishment.
The verse in Surah As-Sajdah concerns disbelievers who deny the truth, not sinful believers in Tawhid.
There is major fisq that takes a person outside Islam and minor fisq that does not take a person outside the religion.
And Allah knows best.