The simple answer is because Islam teaches that we must believe in the Prophets (pbut) and books (Torah and Bible) which came before (in their true form). In Islam Jews and Christians are called People of the Book.
Muslims must believe in all the Prophets from Adam to Mohamed, the Final Prophet, (pbut). This includes Prophet Ibrahim (Abraham - pbuh) and the Quran contains a chapter called Ibrahim, as he is the father of both the Jews and Arabs through his sons.
We believe that Isa (Jesus - pbuh) was sent by G-d to correct the wrongdoings of the Jews (in not following their given faith correctly, hence stories like Jesus fighting with money lenders in the temple and the golden calf). Prophet Mohamed (pbuh) was then sent to correct the wrongdoings of Christians in creating the trinity and placing partners with G-d but the religions, when they were each revealed, were one and the same.
All three religions come from the same source, G-d, and all three state there is only One G-d.
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How strange it is the second time I have met up with this piece of video trash today so I shall just copy and post my answer again.
The start of the video sets a scene of how all religions, other than Islam, are peaceful and the Torah/Bible is full of peace and quaint stories not to be taken literally ... it's effective propaganda but total rubbish of course:
Ezekiel 9:6 "Slay utterly old and young, both maids, and little children, and women . . . "
Isaiah 13:16 "Their children also shall be dashed to pieces before their eyes; their houses shall be spoiled, and their wives ravished."
Deuteronomy 13:15 "Thou shalt surely smite the inhabitants of that city with the edge of the sword, destroying it utterly . . . "
The video deals with 3 topics, all of which are factually incorrect:
1. Abrogated passages - There are 2 issues with this. First, yes later passages of the Quran abrogate newer passages but even the best Islamic scholars can only use their judgement as to when each passage was written, this absolute knowledge (of when each passage was written) was lost centuries ago. Secondly the video suggests that violent passages abrogate peaceful ones ... which violent passages are they speaking of? Usually it is the "sword verses" and every Muslim knows those verses are speaking about a particular set of events which happened in the time of the Prophet, about a particular enemy tribe and are simply used by jihadists to cause violence and by anti Muslims to say how bad Islam is.
2. Worldwide Sharia law - this threat is always used to scare people. All Muslims know that there will be worldwide Sharia law but this will happen when the Messiah returns to earth and wages war against sin. People cannot perform the work of the Messiah and as he hasn't returned yet it means there can't be worldwide Sharia law.
Also the Sharia court in the UK is allowed to rule on matters such as inheritance and divorce. There have been Jewish courts in the UK for decades but I haven't seen you posting video's about that.
3. Taqiyya (lying) is forbidden in Islam, other than in 3 circumstances.
Humaid b. 'Abd al-Rahman b. 'Auf reported that his mother Umm Kulthum daughter of 'Uqba b. Abu Mu'ait, and she was one amongst the first emigrants who pledged allegiance to Allah's Apostle (may peace be upon him), as saying that she heard Allah's Messenger (may peace be upon him) as saying: A liar is not one who tries to bring reconciliation amongst people and speaks good (in order to avert dispute), or he conveys good. Ibn Shihab said he did not hear that exemption was granted in anything what the people speak as lie but in three cases: in battle, for bringing reconciliation amongst persons and the narration of the words of the husband to his wife, and the narration of the words of a wife to her husband (in a twisted form in order to bring reconciliation between them).
There is nothing that allows a Muslim to lie about what the Quran says to make people convert or show Islam in a good light to non Muslims.
If you scroll down on the info bit under the video it gives all the sources it used.
Info about abrogation in the Quran comes from the Methodist Publishing House and a website called Muhammadanism.org ... which they themselves state is very anti-Islam.
Info on Sharia comes from Wafa Sultan, who is a proven liar and she isn't even a Sunni Muslim (the vast majority of Muslims) but Shia, who follow some very strange practices and religious laws not accepted in Sunni Islam. She went to America and became very rich and famous for slagging off Islam after telling a lot of lies about her life back home - she went from welfare in America to a mansion based on these stories so not a very reliable source.
The original video was posted on a blog which claims "Islam is of the devil" ... so no axe to grind there then lol.
A lot of their info originates on a website called Crusader's Armory ... which just about says all we need to know about this video.
