The Injeel is the holy book, or Gospel, that Allah revealed to Prophet Eesa (Peace Be Upon Him). In today’s world this book does not exist, but there are parts and influences from it found in the New Testament and other Gospels. The Injeel IS NOT the New Testament because the New Testament was written after Eesa (Referred as Jesus, Peace be upon him) and by 4 main writers (amongst many) - Peter, Mark, Luke & John. 
The word "Injeel" is derived from the old Greek word euangelion meaning "good news" or "glad tidings". The other books that were revealed by Allah apart from Injeel are the Quran, the Zabur, the Torah, and the Scriptures of Ibrahim and Musa (Peace be upon them). [a]
Allah says: And We sent, following in their footsteps, Jesus, the son of Mary, confirming that which came before him in the Torah; and We gave him the Injeel (Gospel), in which was guidance and light and confirming that which preceded it of the Torah as guidance and instruction for the righteous. Quran Surah al-Ma’idah 5:46
Allah says (interpretation of the meaning): “So for their breaking of the covenant We cursed them and made their hearts hard. They distort words from their [proper] usages and have forgotten a portion of that of which they were reminded. And you will still observe deceit among them, except a few of them. But pardon them and overlook [their misdeeds]. Indeed, Allah loves the doers of good(al-Muhsineen).” Quran Surah al-Ma’idah 5:13
The phrase “because of their breach of their covenant, We cursed them” refers to the fact that they broke the Covenant which had been made with them, so Allaah cursed them, i.e., He kept them away from following the True Guidance. “[We] made their hearts grow hard” means that they will not benefit from any preaching because their hearts are so hard. “They change the words from their (right) places” means that they play havoc with the words of Allaah and misinterpret His Book, taking it to mean things that were never meant and attributing to Allaah things that He never said; may Allaah protect us from that. 
Moreover, these books which were written after the time of the Messiah did not remain in their original form. The original versions were lost long ago.
Ibn Hazm said: With regard to the Christians, there is no dispute among them or anyone else that only one hundred and twenty men believed in the Messiah during his lifetime… and all of those who believed in him concealed themselves and were afraid during his lifetime and afterwards; they called people to his religion in secret and none of them disclosed himself or practiced his religion openly, because any of them who was caught was executed.
They continued in this manner, not showing themselves at all, and they had no place where they were safe for three hundred years after the Messiah was taken up into heaven.
During this time, the Gospel that had been revealed from Allaah disappeared, apart from a few verses which Allaah preserved as proof against them and as a rebuke to them
Then when the Emperor Constantine became a Christian, then the Christians prevailed and started to practise their religion openly and assemble in safety.
If a religion is like this, with its followers practicing it in secret and living in constant fear of the death, it is impossible for things to be transmitted soundly via a continuous chain of narrators and its followers cannot protect it or prevent it from being distorted. Al-Fasl, 2/4-5 
Changing Omitting Adding things and attributing to Allaah words that He did not say, Misinterpreting the words of Allaah.
When Muhammadﷺwas sent, the original Tawraat and Injeel had already been altered and distorted. Allaah revealed the Qur’aan to His Prophet Muhammadﷺ, and guaranteed that He Himself would preserve it, as He says (interpretation of the meaning): “Verily We: it is We Who have sent down the Dhikr (i.e., the Qur’aan) and surely, We will guard it (from corruption).” Quran Surah al-Hijr 15:9
This distinguishing feature was not found in any Book before the Qur’aan. From the time of Muhammadﷺuntil the present (more than fourteen hundred years) the Qur’aan has stayed the same, and not a single letter of it has been changed, as ancient manuscripts and the hearts of generation after generation of people who have memorized the entire text and earned the title of “haafiz” bear witness. May Allaah guide us to the true and straight path. 
Shaykh al-Islam Ibn Taymiyah said in al-Jawaab al-Saheeh (3:21):
With regard to the Gospels that the Christians have, there are four Gospels, Matthew, Mark, Luke and John. They are agreed that Luke and Mark did not see the Messiah, rather he was seen by Matthew and John. These four accounts which they call the Gospel, and they call each one of them a Gospel, were written by these men after the Messiah had been taken up into heaven. They did not say that they are the word of God or that the Messiah conveyed them from God, rather they narrated some of the words of the Messiah and some of his deeds and miracles. 
A Muslim must believe in all the Heavenly Books and he must believe that they are from Allah. It is not lawful for him to abide by its laws, since these Books were revealed to certain nations at certain times.  So Quran is the final book for entire mankind which is to be followed. Allah said: “Verily, We have sent down to you (O Muhammad) the Book for mankind in truth. So whosoever accepts the guidance, it is only for his own self; and whosoever goes astray, he goes astray only for his own loss.”Qur’an Surah Zumar 39:41 
And Allaah knows best.
  http://islamqa.info/en/2001
  http://islamqa.info/en/47516