Question: On what basis do Evangelical apologists argue that Muhammad committed sin?
Romans 5:12 tells us that through Adam sin entered the world, and so death was passed on to all men because “the wages of sin is death” (Romans 6:23). King David understood his sinful nature: “Surely I was sinful at birth, sinful from the time my mother conceived me” (Psalm51:5). Christians acknowledge that we are all natural born sinners and the Bible confirms this: “For all have sinned and fall short of the glory of God” (Romans 3:23). That would include Muhammad. The only sinless person was Christ Jesus: “He committed no sin, and no deceit was found in his mouth” (1 Peter 2:22). That is the biblical and Christian understanding of sin – we ALL sin, and there is no exception.
The difference between the Christian understanding of sin and the Muslim understanding of sin is significant. Christians believe that sinners have broken God’s law (1 John 3:4); sinners are slaves to sin (John 8:34); they face the judgment of God (Jude 1:14–15); they are on the road to death and destruction (Ezekiel 18:20; James 1:5). The ONLY solution to sin is to place our faith in what God, through Christ Jesus, has done – He laid down his life to pay the price of sin, then was raised from the dead and is now seated at the right hand of God.
Muslims do not believe this. They deny that Jesus died and was resurrected. They do not believe in original sin or that every person since Adam has inherited sin.
In Islam, a sin is any act of disobedience against the commands of Allah, which may entail punishment in the Hereafter. In contrast, a good deed (if done sincerely and accepted by the Almighty Allah) will entail reward in the Hereafter. The crucial piece is that sins vs. good deeds are weighed, and the human being is judged on the difference; that determines where you end up. Sin is not a state of being (no "Original Sin"). Sin is an action that you commit. Sins can be committed accidentally, although you are not held accountable for them in that case. You can think of committing sins, but you are only held accountable for actions, not thoughts. https://islam.stackexchange.com/questions/351/what-is-the-muslim-definition-of-sin
Muslims believe they can atone for their own sins by good works. If they comply with the Five Pillars of Islam and perform more good deeds than bad deeds, then Allah may be merciful and they will not go to hell. With regard to Muhammad, I posted a question on the Islam site and received this answer:
The theological Islamic position is that prophets can not commit sins. What is attributed to e.g. Noah, Abraham, Moses عليه السلام etc. are not actual sins - rather excusable or lawful matters over which they repent out of their extreme piety and because of holding themselves more strictly accountable in view of their status and relationship with Allah.
47:19 does not mention a sin of the Prophet Muhammad ﷺ and if you look at its exegesis there are various explanations on whether it is addressed to the Prophet or an ummati, and in which of their possible meanings the words غفر (forgive) and ذنب (sin) are used.
Regarding the incidents over which the prophets repented: One is not necessarily attributed to all prophets, and Jesus عليه السلام is among those prophets regarding whom nothing of the sort is mentioned in the Quran. Although from hadith it may be construed that he is concerned over people committing shirk using him.
The following answer is in response to another question on the Islam site regarding Jesus/Isa being without sin. This makes reference to the Hadiths (traditions): https://islam.stackexchange.com/questions/215/was-the-prophet-sinless/52867#52867
I hope this will help readers to understand the theological differences that separate the Christian view of sin, repentance, forgiveness and salvation from the Muslim view. Muslims might believe that Muhammad never sinned, but their view of sin differs from the biblical view of sin. Only Christ Jesus was without sin. He died so that we might live:
For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God (2 Corinthians 5:21).
P.S. There is a religious movement that attempts to merge Islam and Christianity. It is known as ‘Chrislam’:
While it is undeniable that there are many similarities between Christianity and Islam (and Judaism, for that matter), Chrislam ultimately fails because Christianity and Islam are diametrically opposed on the most important of issues – the identity of Jesus Christ. True Christianity declares Jesus to be God incarnate. For Christians, the deity of Christ is a non-negotiable, for without His deity, Jesus’ death on the cross would not have been sufficient to be the atoning sacrifice for the sins of the entire world (1 John 2:2).
Islam adamantly rejects the deity of Christ. The Qur’an declares the idea that Jesus is God to be blasphemy (5:17). Belief in the deity of Christ is considered shirk (“polytheism”) to Muslims. Further, Islam denies the death of Christ on the cross (4:157–158). The most crucial doctrine of the Christian faith is rejected in Islam. Source: https://www.gotquestions.org/Chrislam.html