How Abu-Abd Allah came to understand the Quran: by inspiration and by practical application.
God let him experience about the same situation and he would then understand the implication of the ayat of the Quran.
What people had said to God's messenger, the same they said to Abu Abd-Allah .. in fact, they used to say the same words to every messenger or apostle.
Quran 42: 43, which means:
(No [mockery or denial] that has been said to you [Mohammed], but it had been said to the messengers [: apostles] before you f)
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43 f It means: Whatever the associaters [and idolaters] have said to you in answer to your inviting them, is like to the denial and mockery that had been said to the other messengers before you; so do not bother yourself in case they deny you and do not be sad in case they ridicule you; for this is not a new one of their acts.
http://quran-ayat.com/pret/41.htm#a41_43