@josephdiwan In reference to answer the Islam side of this question, there were differences in the belief. Muslims feel that there were no alterations to the quran and it is in its forever state, but with a little history lesson, they are actually wrong.
I copied and pasted some notes from a book by Josh McDowell called Understanding Islam and Christianity, it may help you in your defense.
Muhammad began to proclaim the Quran following the angel Gabriel’s first visitation. This occurred when he was about 40 (AD 610), and Quranic revelation ended when he died 23 years later (AD 633). Throughout this time he claimed to receive the various passages that comprise the Quran from Gabriel. 2. The Quran today consists of 114 chapters, or “suras,” generally arranged in order of length. The Quran is not arranged chronologically or arranged according to topic. Suras are named after key figures or subjects or simple references found in the chapter. For example, the sura named “Women” is named so because much of the sura deals with women. 3. Scholars do not agree on the exact chronological order of the Quran. However suras 9 or 5 are often identified as the last sura while the first passage of the Quran to be revealed is the first part of sura 96. 4. “Quran” means “Recitation” or “Reading” in English. 5. The 23-year period of revelation occurred in both Mecca and Medina. Most of the individual suras were revealed in Mecca. The longest suras were revealed in Medina. Sura 2 is a reflection of Muhammad’s conflicts with the Jews of Medina. 6. Most suras consist of verses revealed in one respective town (Mecca or Medina) or the other, but some suras contain a mix of passages revealed in both Mecca and Medina. 7. Many Muslim scholars believe that the Quran is an uncreated, eternal attribute of Allah. It is the final revelation from God to man. 8. The Traditions (Hadith) and Biographical (Sira) literature are additional Islamic source texts which are used to establish Islamic theology but the Quran stands above those. 9. Muhammad often received Quranic revelations in response to immediate challenge, conflict, or controversy and in this the Quran was revealed as guidance to aid him. The Quran does not usually provide the context behind those revelations (this is where the Hadith and Sira help greatly). An entire branch of Islamic theological study, “Asbab al-nuzul” (reasons or circumstances of the revelation), exists to provide passages’ context. 10. The Quran addresses many themes including Allah’s greatness, prayer, alms, points based on biblical and apocryphal Jewish and Christian stories, theological conflicts, military battles, civil relations, interactions with non-Muslims, marriage, heaven, hell, slavery, and so forth. 11. There were dozens of early compilations of the Quran. Although they were fairly uniform and had much in common there were also differences: Some had more suras, some had less, verses had significant differences (more than just pronunciation or dialect differences), verses were missing, different words were used, and so on. Despite these differences, none of Islam’s theological tenets are threatened.
Below are several categories of evidence that prove that today’s Quran is not the supernatural, pristine text many Muslims believe it to be. We cannot go into each category in great detail but we will provide references for your further study. 1. Quran Variants During Muhammad’s lifetime his companions wrote down various parts of the Quran on scraps of parchment, bones, stone, and so on. Following Muhammad’s death some compiled their own collections of what they knew of the Quran. Also following Muhammad’s death, during Abu Bakr’s reign, thousands of people who were coerced into becoming Muslims while Muhammad lived rebelled and left Islam. Abu Bakr sent armies out to either kill these people or have them reconvert to Islam (these were called the “Wars of Apostasy”). Great battles were fought and thousands on each side died. Following that, Abu Bakr realized that so many men who had memorized portions of the Quran had died and thus was worried that part of the Quran might have been lost. He asked Zaid, a “companion” of Muhammad, who was capable and knowledgeable of the Quran, to undertake its compilation. Zaid commented that this was an incredibly difficult task but would undertake it nonetheless. Bukhari’s Hadith, volume 6, number 509 details all of this. However, other prominent Muslims, such as Ibn Masud and Ubay ibn Kab (and many others), had also compiled their own versions of the Quran. Some, like Ibn Masud, taught Quranic schools in different Islamic regions and propagated the use of their own versions of the Quran. As Islam spread, conflict arose as to which Quran was correct. Muslims were ready to fight with other Muslims over the differences. This prompted the third Caliph, Uthman, to set into motion the compilation of an official Quran. He asked Zaid and several other Muslims to do the work. Once Zaid’s work was finished, Uthman had several copies made and sent to the primary Islamic provinces. But more importantly, Uthman ordered that all other competing Quran versions be burned and destroyed. Bukhari’s Hadith, volume 6, number 510 details this. This destruction did not sit well with the other notable Muslims who had compiled their own Qurans. Prominent “companion” Muslims, like Ibn Masud, protested and said they knew more of the Quran than Zaid. Some stated that Zaid’s Quran was inaccurate and contained flaws. Further they declared that Uthman was destroying Allah’s word! Nevertheless, Uthman was the ruler and his command was put into effect. Those that disobeyed were punished.
The dispute between books of the bible today with the 66 books is a long answer with church history going back as far as Martin Luther and even further. In sum the bible is 66 books written by over 40 authors over 1500 years and it tells one story “God’s love and redemption for mankind” Jesus is the center point of the entire bible.
If someone is questioning you, it seems as if they are attempting to point out faults with the Christian faith on topics such as the council of trent, or council of nicea or other issues in the church.
Besides the Islamic religion developing 610 years after Christ, and the points at the bible, I think the best defense to the Christian faith is always this "The bible is not a deck of cards where if you pull one card out, and all the other cards come with it. The bible is not the starting point to the christian faith. It never was. In the time of Jesus there was no bible (old and new testament). In that time and forever the starting point to the Christian faith was an EVENT. That event is Jesus raising from the dead. You see if anyone could disprove that event, then the whole Christian faith is false, but if that event is true and can’t be disproved, then everything else in the bible is true to. From the famous words of Andy Stanley “I don’t believe the bible because the bible says its true. I believe the bible because Jesus says its true. And any man that could predict his own death and resurrection, and pull it off, then I will just believe whatever that man has to tell me.”