God has declared in the Qur’an that all the prophets and their peoples—from Noah down to the disciples—were Muslims, true believers, as has been explained elsewhere.
He, the Exalted, says:
“He has ordained for you, as religion, what He enjoined upon Noah, and what We have revealed to you, and what We enjoined upon Abraham, Moses, and Jesus: ‘Establish the religion and do not become divided concerning it.’” (Q 42:13)
Thus the messengers were commanded to uphold the religion and not to fall into schism. These five are the Messengers of firm resolve; God mentions them together in two places in His Book—here, and in His words:
“When We took from the prophets their covenant, and from you, and from Noah, Abraham, Moses, and Jesus son of Mary; and We took from them a solemn, binding covenant.” (Q 33:7)
He, Most High, also says:
“O messengers, eat of the good things and act righteously—surely I am well aware of what you do. Truly, this community of yours is one community, and I am your Lord, so be mindful of Me. Yet they split their affair among themselves into sects, each party rejoicing in what it possessed.” (Q 23:51–53)
And He says:
“So set your face toward the religion as a ḥanīf, in accord with the natural disposition with which God created humankind. There is no changing God’s creation—this is the upright religion, though most people do not know—turning back to Him, and fear Him, and establish prayer, and do not be among the polytheists: those who divided their religion and became factions, each party pleased with what it had.” (Q 30:30–32)
He further says:
“Mankind were but a single community.” (Q 10:19)
Likewise, He says in Sūrat al-Baqarah:
“Then God sent the prophets as bearers of glad tidings and warners, and He sent down with them the Scripture in truth so that it might judge between people concerning that over which they differed. None differed about it except those who had been given it, after clear proofs had come to them, out of mutual envy and transgression. Then God guided those who believed to the truth concerning that over which…” (Q 2:213)