What is Islam?
The Jewish community is the most similar to the Muslim community in both theology and devotional practice, as indicated by the sound hadith (saying of Prophet Muhammad(pbuh)), “You are most like the Children of Israel.” In fact, there are only a few main differences between the two Abrahamic religions, Islam and Judaism. Almost everything else they have in common. To many people's surprise, Islam and Judaism has more in common and fewer differences than Christianity and Judaism does. For example, the strict belief in the oneness and unity of God, the strict monotheism. Whereas mainstream Christianity has the belief in a triune god: God the Father, god the son, and god the holy spirit. Christians believe that God the father incarnated into the body of Christ. Both Judaism and Islam view incarnations of God into a human being as idolatry. Some of the other similarities between Judaism and Islam are the beliefs in the Prophets, Angels, Day of Judgment or Resurrection, Heaven (Garden of Eden) and Hell (Gehennim), etc... Another similarity is the belief that when people sin, all they must do is repent to God (Allah in Arabic or Elohim in Hebrew) and not commit the sin again, and then God will forgive them. As it says in Hosea 6:6, "For I desired mercy, and not sacrifice: and the knowledge of God more than burnt sacrifice".
Islam believes that the religion and message of all of the Prophets from Adam was one and the same. And that whomever followed the Prophet of their time period were Muslims (those who submit to God's will). So for example, God chose Abraham and his lineage to preach the message of the One God to humanity. This continued through Isaac and Jacob and the Children of Israel until Prophet Jesus came. And then the Nazarenes, referred to in Arabic as Nasara, (were the Jewish followers of Jesus who followed the law of the Torah) continued on this mission. Finally the Abrahamic tradition was entrusted to a Prophet from the lineage of Ishmael, Prophet Muhammad, and he continued on in this same message of teaching humanity about the One God of Abraham. Prophet Muhammad never claimed to bring a new religion, but rather he said that he brought the same religion that all the other Prophets brought such as Prophets Noah, Abraham, Ishmael, Isaac, Jacob, Moses, and Jesus. Islam is purely just an extension of the Hebrew Prophetic Tradition.
As it says in the Qur’an 42:13, "The same religion has He established for you as that which He enjoined on Noah - the which We have sent by inspiration to thee - and that which We enjoined on Abraham, Moses, and Jesus: Namely, that ye should remain steadfast in religion, and make no divisions therein: to those who worship other things than Allah, hard is the (way) to which thou callest them. Allah chooses to Himself those whom He pleases, and guides to Himself those who turn (to Him)." Qur'an 22.78: "And strive in His cause as ye ought to strive, (with sincerity and under discipline). He has chosen you, and has imposed no difficulties on you in religion; it is the religion of your father Abraham. It is He Who has named you Muslims, both before and in this (Revelation); that the Messenger may be a witness for you, and ye be witnesses for mankind! So establish regular Prayer, give regular Charity, and hold fast to Allah! He is your Protector - the Best to protect and the Best to help!"
And also in the Qur'an 6:84-87, "We gave him (Abraham) Isaac, and Jacob: all We guided and before him, We guided Noah, and among his progeny David, Solomon, Ayub, Joseph, Moses, and Aaron :Thus do We reward those who do good. And Zakariya and Yahya, and Jesus and Elias: all in the ranks of righteous. And Isma'il and Elisha and Jonah (Yunus) and Lut. And to all we gave favor above the nations. (To them) and to their fathers, and progeny and brethren (the Bani Israel): We chose them, and we guided them to a straight way." We also find in the Qur'an 29:27, "And We gave (Abraham) Isaac and Jacob, and ordained among his progeny Prophethood and Revelation, and We granted him his reward in this life; and he was in the Hereafter (of the company) of the Righteous." And also in Qur'an 45:16, "And verily we gave the Children of Israel the Scripture and the Command and the Prophethood, and provided them with good things and favored them above (all) peoples;"
The reason Prophet Muhammad came was that; first he taught that the previous revelations (the Torah, Psalms, and Gospel) and the majority of the Prophets had been only directed at the Children of Israel, while his message was a universal one. As it says in Sahih Bukhari Volume 4, Book 56, Number 661: Narrated Abu Huraira: The Prophet said, "The Israelis (Bani Israel) used to be ruled and guided by Prophets: Whenever a Prophet died, another would take over his place." Also in Bukhari Volume 1, Book 7, Number 331: Narrated Jabir bin 'Abdullah: The Prophet said, "I have been given five things which were not given to any one else before me.
1. Allah made me victorious by awe, (by His frightening my enemies) for a distance of one month's journey.
2. The earth has been made for me (and for my followers) a place for praying and a thing to perform Tayammum (washing with sand or earth if water is not available), therefore anyone of my followers can pray wherever the time of a prayer is due.
3. The booty has been made Halal (lawful) for me yet it was not lawful for anyone else before me.
4. I have been given the right of intercession (on the Day of Resurrection).
5. Every Prophet used to be sent to his nation only but I have been sent to all mankind.
Imam Ahmad bin Hanbal in his Musnad has mentioned a Hadith (no. 21257) that Allah sent 124, 000 Prophets (Anbiyah') and from among them 315 were the Messengers (rasul). Abu Tharr (radhiyallaahu 'anhu) asked the Messenger of Allah (sallallaahu 'alayhe wa salaam) about the number of prophets. He said: "124,000 and from them is a group of 315 messengers."
The other reason was that he said that certain portions of the Jewish and Christian teachings or interpretations of their scriptures had become corrupt and that he had come to restore the faith to the pristine purity of the Faith of Abraham. With that being said, Muslims are to believe in whatever is said in the earlier revelations (Torah and Hebrew Prophets) provided they don't contradict with what is in the Qur'an or the authentic Hadith (the sayings and actions of the Prophet). According to Islam, the only reason for the contradiction would be that that portion of the earlier scripture had been corrupted. But besides those discrepancies, ALL else is to be believed. It says numerous times in the Qur'an, that the Qur'an came to CONFIRM the previous scripture, NOT to REPLACE the previous scripture. Muslims don't believe that Islam started with Prophet Muhammad, in fact, the Qur'an says that Prophet Ibrahim is the father of the Muslims, thusly Islam believes in successive revelations, the Book of Abraham, the Torah of Moses, the Zabur (Pslams) of David, and the Injil (Evangel) of Jesus. The Qur'an came to confirm the previous revelations and make the message Universal. According to Islam, previously, Allah had focused his covenant on the Children of Israel, now He was making his covenant universal.