Allah, the Exalted, says: “And [remember] when Allah took the covenant of the prophets: ‘Whatever I give you of the Book and wisdom, then there comes to you a messenger confirming what you already have, you must believe in him and support him.’ He said, ‘Do you affirm and accept My covenant on that condition?’ They said, ‘We affirm.’ He said, ‘Then bear witness, and I am with you among the witnesses.’” ⦗3:81⦘.
The messenger who came confirming what the earlier messengers possessed is Muhammad ﷺ, as Allah also says: “We have sent down to you the Book in truth, confirming what preceded it of the Scripture and as a guardian over it” ⦗5:48⦘; and: “He it is who sent His Messenger with guidance and the religion of truth so that He may cause it to prevail over every religion” ⦗9:33⦘. This prevalence is both a matter of divine decree and of revealed law.
Accordingly, anyone to whom the message of the Prophet ﷺ reaches but who neither believes in him nor follows him is neither a believer nor a Muslim; rather, such a person is an unbeliever destined for Hellfire. The Prophet ﷺ said: “By Him in whose hand is Muhammad’s soul, no one of this ummah—meaning all whom the call reaches—whether Jew or Christian, hears about me and then dies without believing in what I was sent with except that he will be one of the inhabitants of the Fire.” Muslim recorded this on the authority of Abu Hurayrah (may Allah be pleased with him).
Hence it is clear that the controversy over whether the former nations were ‘Muslim’ or not is only a dispute about words. In its universal sense Islam encompasses every valid law with which Allah sent a prophet, and thus it includes the Islam of every community that followed its prophet so long as his law remained in force and had not been abrogated, as the previously cited texts affirm. After the commissioning of Muhammad ﷺ, however, Islam is confined to what he brought. Whoever does not believe in him and follow him is not a Muslim, and anyone who claims that alongside Muhammad’s religion there exists another faith—whether Judaism, Christianity, or anything else—still valid and acceptable to Allah is refuted by His words: “Indeed, the religion in the sight of Allah is Islam” ⦗3:19⦘, and: “Whoever seeks a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers” ⦗3:85⦘.