The Religion that was revealed by Allah is Islam
"Islam" within this context points to all revelations as the verse lately explains:
People who were given the Book differed after they received the knowledge, for they were unjust to each other (HQ: 3:19).
Those "People of the Book" "differed" among themselves, not because Revelations were different but because they were not following the righteous path. Despite the fact that they received that knowledge of that Religion through that one "Book" as revealed in various times, they disputed and lived in conflict. The reason for that is because they "were unjust to each other" as the phrase in Arabic bagheyan baynahum indicates.
To elaborate, it is interesting to observe that the word "Islam" is derived from the Arabic root: s-l-m. Out of this root, the noun salam "peace", the verb sallama "to surrender" and the adjective salem "the saved one", are also derived, while bagheyan is derived from b-gh-y. As one word, baghy means an "unjustified" attack. Those who differ cannot coexist with each other; they quarrel and dispute without justification. That kind of conflict is in itself an indication of the deviation from the holy guidance. As such, people lost the ability to discern between what is true and what is false. In contrast to those people, Muslims (with the broad meaning of the word) are those who surrender to the holy, and therefore they live in peace within and manifest peace around. The holy verse to follow explains it. The Holy Qur'an addresses The Prophet (PBUH):
So if they dispute with thee, say: "I have submitted my whole self to Allah and so have those who follow me." And say to the People of the Book and to those who have not received holy guidance: "Do ye (also) submit yourselves?" If they do, they are in right guidance, but if they turn back, thy duty is to convey the Message; and in Allah's sight are (all) His servants (HQ: 3:20).
The Prophet (PBUH) was asked to convey the Message of Allah by being an exemplar, and not by imposing anything on others. Peace, not conflict was what he was manifesting. He said about himself,
"O People, I am Mercy that is presented to you (by Allah)" (PH)
Narrated by Al-Darmi
He called for that One Religion by uncovering the oneness of Revelations to prophets who came before him:
"The parable of prophets who proceeded and myself is like a building that someone had beautifully and perfectly constructed. But, a single brick was missing in one of its corners. So, whenever people passed by that building, expressing their facination by it, they used to say, 'the missing brick has to be put'. I am ( the similitude of) that brick. And I am the Seal of prophets"
Narrated by Al Bukhari and most scholars of Hadith.
Namely, The Prophet (PBUH) explained to his followers that believing him implied believing all previous prophets and Revelations. The Holy Qur'an expresses this truth:
The Messenger believeth in what hath been revealed to him from his ord, as do the men of faith. Each one (of them) believeth in Allah, His angels, His Books, and His Messengers. We make no distinction between one and another of His Messengers. And they say: "We hear, and we obey, (we seek) Thy forgiveness, our Lord, and to Thee is the end of all journeys (HQ: 2:285).
The Holy Qur'an states that Allah reveals the same truth to all prophets, those who are mentioned, and others who are not mentioned in the Holy Qur'an:
We have sent thee inspiration, as We sent it to Noah and the Messengers
after him: We sent inspiration to Abraham, Isma'il, Isaac, Jacob, and the Tribes,
to Jesus, Job, Jonah, Aaron, and Solomon, and to David We gave the Psalms. Of some
Messenger We have already told thee the story; of others We have not, and to Moses
Allah spoke direct
Because there is only One Religion, God asks those who follow The Prophet (PBUH) to say,
"We believe in Allah, and the revelation given to us, and to Abraham, Isma'il, Isaac, Jacob, and the Tribes, and that given to Moses and Jesus, and that given to (all) Prophets from their Lord: we make no difference between one and another of them: and to Him we surrender (become Muslims)" (HQ: 2:136).
The guidance about the belief in the one Religion revealed to all prophets was primarily directed to The Prophet Muhammad (PBUH) by the Divine:
Say: "We believe in Allah, and in what has been revealed to us and what was revealed to Abraham, Isma'il, Isaac, Jacob, and the Tribes, and in (the Books) given to Moses, Jesus, and the Prophets, from their Lord: we make no distinction between one and another among them, and to Allah do we bow our will (in Islam)" (HQ: 3:84).
In the language of the Holy Qur'an, prophets are called Muslims, so were their followers. This stands as an indication to the One Religion:
Abraham was not a Jew nor yet a Christian; but he followed the Primordial Religion and hence was a Muslim, and he took not gods with Allah (HQ:3:67).
It is the faith of your father Abraham. It is He Who has named you Muslims, both before and in this (Revelation) (HQ: 22: 78).
Jesus' disciples are recorded in the Holy Qur'an as called themselves Muslims, when Jesus said, "Who will support me in the Message of Allah,?" "We are supporting Allah's message with you, we believe in Allah, and do thou bear witness that we are Muslims", said the disciples" (HQ: 3:52).
And behold! I inspired the Disciples to have faith in Me and Mine Messenger; they said, `We have faith, and do thou bear witness that we bow to Allah as Muslims'" (HQ:5:111).
Moses u addressed his people saying,
"O my people! If ye do (really) believe
in Allah, then in Him put your trust if ye submit (your will to His and become Muslims),"
The Magicians who followed Moses called themselves Muslims: our Lord! pour out on us patience and constancy, and take our souls unto Thee as Muslims (who bow to Thy Will)!" (HQ: 7:126).
Noah u said,
"But if ye turn back, no reward have I asked of you: my reward is only due from Allah and I have been commanded to be among (the Muslims) those who submit to Allah's Will" (HQ: 10:72).
"..and knowledge was bestowed on us in advance of this, and (that is because) we have been Muslims." (HQ:27:42)
Joseph u called the Lord to bless him by making him submit to Allah's will as a Muslim till the end of his life on the earth:
O my Lord! Thou hast indeed bestowed on me some power, and taught me something of the interpretation of dreams and events, O Thou creator of the heavens and the earth! Thou art my Protector in this world and in the Hereafter. Bless me by enDeeng my journey (on earth) as a Muslim, and unite me with the righteous (HQ: 12:101).