A Muslim asked "Why are Christians allowed to defend Christianity in a presuppositional way, but Muslims are not allowed to do the same?
You can try to do so; but it is contradictory to logic, because Islam claims to be the fulfillment of the OT and NT and comes more than 5 Centuries after the NT events, and yet, contradicts the teachings of the Bible, therefore, Islam is proven wrong.
Simple answer: Because Islam came almost 600 years later; yet contradicts the Bible in key areas. Because Islam has no historical evidence to back up its claims that it is a fulfillment of the OT and NT; yet claims to affirm the OT and the NT, and yet contradicts what the OT and NT teach.
The Jews of the NT - Jesus and His disciples/apostles were steeped in the OT and quoted from it extensively.
The Arabs and Muhammad did not quote from the OT (there is one allusion; Qur'an 5:45), and did not know much of the content of the NT, called the "Injeel" (a corrupted form of the word, "Evangel".) (except that it existed, and was the revelation/message of Jesus, and that Jesus had faithful disciples who were honest; and that Jesus was Al Masih (The Messiah), virgin born, and did miracles, and is the son of Mary.
Jay Smith demonstrates that the Bible does not refer to Muhammad, the founder of Islam, even though the Muslims are forced to try to find him in the previous Scriptures, the Torah (The law of Moses; Arabic - Taurat تورات) and the Injeel انجیل (Gospel), because of 2 verses in the Qur'an. (Surah 7:157; Surah 61:6). See more in this series from Jay Smith, highlighted by AnsweringMuslims.com.
Surah 7:157"Those who follow the Messenger, the unlettered prophet, whom they find written in what they have of the Torah and the Gospel, who enjoins upon them what is right and forbids them what is wrong and makes lawful for them the good things and prohibits for them the evil and relieves them of their burden and the shackles which were upon them. So they who have believed in him, honored him, supported him and followed the light which was sent down with him - it is those who will be the successful." (Sahih International Version)
"And [mention] when Jesus, the son of Mary, said, "O children of Israel, indeed I am the messenger of Allah to you confirming what came before me of the Torah and bringing good tidings of a messenger to come after me, whose name is Ahmad." But when he came to them with clear evidences, they said, "This is obvious magic." (Sahih International Version.) See here for the Arabic and English.
The 3 main passages Muslims use to try and claim that Muhammad is prophesied there, are:
1. Deuteronomy 18:15 - this one is very clear in the immediate context of chapter 17 and 18, that the prophet who will come in the future will be a Jew, an Israelite.
"The Lord your God will raise up for a prophet like me, from among your own brothers (same word as "countrymen" in 17:14-15) . . . " Deut. 18:15 (see also Deut. 18:2, "brothers")
(from among your brothers.) ( I cannot figure out how to put the vowel pointing in, but anyone with a Bible original language software program can see it, and perhaps the Hebrew is on the web somewhere.)
“When you enter the land which the LORD your God gives you, and you possess it and live in it, and you say, ‘I will set a king over me like all the nations who are around me,’ 15you shall surely set a king over you whom the LORD your God chooses, one from among your countrymen you shall set as king over yourselves; you may not put a foreigner over yourselves who is not your countryman." Deuteronomy 17:14-15
The context is so clear here, it should be embarrassing for Muslims to try and use this.
Jesus fulfilled Deut. 18:15 in Acts 3:22.
2. Song of Solomon 5:16 - Jay did a good job of defeating that; the context, beginning in verse 10, is Solomon's bride/lover/wife in Jerusalem, talking about Solomon, her beloved. Dr. White also proved this Muslim apologetic method wrong some time ago.
3. John chapters 14 and 16, and the Greek word, παρακλητος (parakletos)
On the Greek word, "parakletos" in John 14:16-17; 14:26; and John 16:7, with 16:12-15, it helps to have a visual of the Greek word in question.
περικλυτος - the word the Muslims claim was originally there in Greek. In order to Muslims to make it work, they have to change 3 key letters of the word. Jay Smith makes a great point that in Arabic, it is a consonantal language, the verbs are sounds in between the letters and most of the time, they are not even written. The Muslim apologists, many times, try to equate Greek with Arabic and say that the vowels in Greek could have been different. They are using an Arabic language characteristic and imposing it upon the Greek language.
"I will ask the Father, and He will give you another Helper, that He may be with you forever; 17 that is the Spirit of truth, whom the world cannot receive, because it does not see Him or know Him, but you know Him because He abides with you and will be in you."
"Helper" - is the translation of the Greek, παράκλητον - the accusative form (the accusative is the object of the verb "to give", and in Greek the accusative form changes the last letter in this word in order to show that it is the object of the verb) from παράκλητος (nominative, dictionary form).
"But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you." (John 14:26)
Reasons why it makes no sense for Muslims to use John 14 and 16 and the word "parakletos" to try and prove their apologetic.
1. There is no manuscript evidence and no textual variant of this word in John 14 or 16, so for Muslims to just make up the changes in the three letters is a massive problem, and many times downright deceptive. Parakletos means "the one called alongside" to help or comfort or advocate for; "the comforter", "the helper", "the counselor". The Greek word does not mean "praised one" or "glorious one". (I don't understand Jay Smith saying it means "glorious one", when "Ahmad" احمد (another form of Mohammad- محمد , "the praised one" - you can see the root, hmd, حمد , which means "praise" in Arabic.) in the Qur'an 61:6 means "praised one".
2. The parakletos is "the Spirit of truth" who will be in you. Obviously, this is about the Holy Spirit who would baptize believers and indwell the disciples and all believers (John 7:37-39). It cannot be about Muhammad, because Muhammad is in no sense "in" them. The fulfillment of this is clear in Acts chapter 2 and explained in John 7:37-39 and I Corinthians 6:19-20; 12:13, Galatians 4:6; Ephesians 1:13; 4:30 . It cannot be about Muhammad, who was a human coming almost 600 years later, he was not a spirit.
3. Muhammad does not "abide with them".
4. Muhammad cannot be with the disciples "forever".
5. Muhammad came into existence 570 years after the birth of Christ, so the promise makes so sense for the disciples of Jesus at all, much less for other Christians.
6. Muhammad is not a spirit, but a man.
7. Muhammad does not glorify Jesus in the way that the NT means.
8. Muhammad was not sent from "the Father". Muslims object to even calling God, "Father". (Surah 112)
The Presupposition that the Bible is true and the God of the Bible is true conforms to reason and logic. A presupposition needs to make sense; and be in conformity to truth and not contradict evidence, or its own claims. Islam makes no logical sense, since it claims to be a third stage in revealed religions, and that Muhammad came to complete Judaism and Christianity. Islam makes no sense, since Muhammad cannot be the "parakletos", the Helper, the comforter of John 14 and 16. Islam makes no sense, since it affirms the OT and the NT (Surah 2:136; 5:46-48; 5:68; 10:94; 29:46); yet contradicts the content of those same Scriptures. And Islam makes no sense, because it flatly denies the crucifixion and death of Jesus (Surah 4:157); one of the most accepted historical facts of history; and something that even skeptic unbelievers like John Dominic Crossan and Bart Ehrman affirm as historical fact.