Prophet Muhammad (PBUH) knew Arabic because it was his native language; there is no solid evidence that he knew Syriac or studied Syriac scriptures. The presence of shared Semitic words or Arabized foreign terms in the Quran proves contact between languages, not human authorship of revelation.
Why the question sounds strong
The argument is usually put like this: the Quran mentions Biblical figures, uses words that resemble Syriac, Hebrew, Aramaic, or Ethiopic terms, and speaks in a highly literary Arabic. Since Prophet Muhammad (PBUH) was not known to be a reader of scripture, critics ask: where did this knowledge come from? Perhaps, they say, he learned from Jews and Christians, absorbed Syriac religious material, then recast it in Arabic.
That is the strongest form of the objection. It is not silly to notice that Arabic, Syriac, Hebrew, and Aramaic are related Semitic languages, or that Arabia had trade, Jewish tribes, and Christian contacts. The real question is what those facts actually prove.
Arabic was his own language
There is no puzzle about the Prophet (PBUH) knowing Arabic. He was born and raised among the Arabs of Quraysh, in a culture where memory, poetry, public speech, and precise expression mattered deeply. Being "unlettered" did not mean being inarticulate; it meant he was not known to read and write scripture.
You did not recite before it any scripture, nor did you inscribe one with your right hand. Otherwise the falsifiers would have had cause for doubt.
Quran 29:48
This verse does not say he lacked intelligence or language. It says his life did not fit the profile of someone trained in books, copying earlier texts, or preparing a literary-religious project.
Shared words are not secret borrowing
Some Quranic words have cognates in Syriac, Hebrew, Ethiopic, or other languages. Classical scholars discussed this frankly. Some held that every Quranic word is Arabic in usage; others said certain words had foreign origins but became Arabized before revelation. These views are not scandalous. English has words from French, Latin, Arabic, and Hindi, yet a sentence using them can still be English.
The Quran itself answered the claim that a foreign teacher was behind the revelation:
And We certainly know that they say, "It is only a human being who teaches him." The tongue of the one they refer to is foreign, and this is a clear Arabic tongue.
Quran 16:103
The point is not that Arabic had no shared vocabulary with other languages. The point is that the Quran came as a sustained Arabic discourse - legally, spiritually, rhetorically, and theologically - not as the broken product of a foreign instructor.
Syriac knowledge is not established
The strongest reply is simple: we do not have reliable evidence that Prophet Muhammad (PBUH) knew Syriac, read Syriac texts, or studied under Syriac teachers. In fact, when the Muslim community needed foreign correspondence handled, he used scribes and translators. Zayd ibn Thabit reported that the Prophet (PBUH) instructed him to learn the writing of the Jews so that official letters could be managed safely.
The Messenger of Allah (PBUH) commanded me to learn for him the writing of the Jews, and he said, "By Allah, I do not trust the Jews with my letters." So I learned it.
Reported by Abu Dawud and al-Tirmidhi
That report points in the opposite direction of the objection: the Prophet (PBUH) did not personally depend on hidden multilingual literacy. He assigned such tasks to qualified companions.
So what does the argument establish? It establishes that Arabia was not sealed off from Jews, Christians, trade, and neighboring languages. It may also establish that some Quranic vocabulary had wider Semitic roots. What it does not establish is that Prophet Muhammad (PBUH) read Syriac scriptures, copied them, or produced the Quran from human study. The historical evidence for that leap is missing, while the Quran's own Arabic form, public recitation, and the Prophet's known life point elsewhere.