The Prophet—may Allah bless him and grant him peace—said:

“Whoever possesses the means—provisions and a mount that can convey him to the House of Allah—and yet does not perform the pilgrimage, let him die, if he wills, as a Jew or if he wills, as a Christian.”

The Exalted One also says:

“Allah bears witness that there is no deity except Him—as do the angels and those endowed with knowledge—maintaining His creation in justice. There is no deity except Him, the All-Mighty, the All-Wise. Truly, the religion in the sight of Allah is Islam. Those who were given the Scripture differed only after knowledge had come to them, out of mutual envy. Whoever rejects the signs of Allah will find that Allah is swift in reckoning. If they dispute with you, say, ‘I have submitted my whole being to Allah, and so have those who follow me.’ And say to those who were given the Scripture and to the unlettered, ‘Have you submitted?’ If they submit, they are rightly guided; but if they turn away, your duty is only to convey the message. Allah is All-Seeing of His servants.”
(Qur’an, Āl ʿImrān 18–20)

Here Allah, glorified be He, affirms that with Him the religion—first and last—is Islam, a single faith. He then explains that the People of the Book fell into disagreement only after knowledge had reached them, out of envy among themselves, not out of any pursuit of truth.

This is like His saying:

“Those who were given the Scripture did not become divided until after the Clear Proof had come to them. Yet they were commanded only to worship Allah, devoting the religion exclusively to Him, as upright monotheists, to establish the prayer and to give the zakah—and that is the upright religion.”
(Qur’an, al-Bayyinah 4–5)