The Qu'ran does not say that parts have been lost, and I'm sure you believe Allah is powerful enough to preserve his words?
And when were parts of the Injil lost, as it would seem from the Qu'ran that it existed in its entirety while Muhammad was alive.?
The Qu'ran instructs you to look for the straight path. Isa Al-Masih claims to be that the one and the only path. The Qu'ran tells you that He was born of a virgin, A holy son who was given the Word of God. Isa is the glorious One who had been prophecied as the Messiah. Isa is foremost in the world and in the hereafter. He will return to be our judge..... ETC.
Consider who Isa is in this... “. . . Ibrahim said: ‘O my son, I really saw in a dream that I (should) sacrifice you. So think about it. What is your opinion?’ He answered, ‘O my father, do whatever has been commanded to you. . .’ Then both of them surrendered themselves and Ibrahim laid his son on his altar . . . (God) called him, ‘O Ibrahim, truly you have fulfilled that dream . . . and We redeem your son with the offering (of a goat) that is big (great, noble)’”
The Injil tells us that Jesus is the One who takes away the sin of the world. It is He who is our Savior. It is in Him we must place our trust. He Jesus said "I am the Way, the Truth, and the Life. No one comes to the Father except through Me".
I've gone through all these similar argument and completely reject them with the best intentions at heart. God'sunchangingword and people who try to seduce Muslims through twisting the Quran for example Nabil Qureyshi would have to answer for themselves.
Allah is powerful enough to preserve his words but that doesn't mean that every word attributed to God is actually God's words. The Injil exists in even today in its true form and it'll do so forever but the Bible does not showcase the Injil as it is. For starters the Bibles that are commonly used are translations and have been intepreted over generations. The word "son" could well mean "special". So on and so forth. The Injil is incorruptable but not uncoverable or untrustable.
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Know they not Allah Knoweth what they conceal and what they reveal? And there are among them illiterates, who know not the Book, but (see therein their own) desires, and they do nothing but conjecture. Then woe to those who write the Book with their own hands, and then say: 'This is from Allah,' To traffic with it for a miserable price! Woe to them for what their hands do write, and for the gain they make thereby.
(Quran, 2:77-79)"
The Arabic word translated to "illiterate" really means unlearned in previous revelation. So this verse is talking about people like St. Paul
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Here is further Quranic evidence:
"O Apostle! let not those grieve thee, who race each other into unbelief: (whether it be) among those who say "We believe" with their lips but whose hearts have no faith; or it be among the Jews,- men who will listen to any lie,- will listen even to others who have never so much as come to thee. They change the words from their (right) times and places: they say, 'If ye are given this, take it, but if not, beware!' If any one's trial is intended by God, thou hast no authority in the least for him against God. For such - it is not God's will to purify their hearts. For them there is disgrace in this world, and in the Hereafter a heavy punishment. (The Noble Quran, 5:41)"
"But because of their breach of their covenant, We cursed them, and made their hearts grow hard; they change the words from their (right) places and forget a good part of the message that was sent them, nor wilt thou cease to find them-barring a few - ever bent on (new) deceits: but forgive them, and overlook (their misdeeds): for God loveth those who are kind. (The Noble Quran, 5:13)"
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The Quran explicitly states to judge the former scriptures by the Quran to know the truth
"To thee We sent the Scripture in truth, confirming the scripture that came before it, and guarding it in safety: so judge between them by what God hath revealed, and follow not their vain desires, diverging from the Truth that hath come to thee. To each among you have we prescribed a law and an open way. If God had so willed, He would have made you a single people, but (His plan is) to test you in what He hath given you: so strive as in a race in all virtues. The goal of you all is to God; it is He that will show you the truth of the matters in which ye dispute.
5:48
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In a verse Allah states that falsehood cannot come before or after the Quran which means that it did for the previous revelation. That is they added uninspired books before and after the true word of God. I seem to have forgotten the reference and cannot find it online.