No matter how hard a messenger of Allah may try to convert everyone into a true believer, most of them are not true believers and they never will be. This shows that the guidance is only from Allah and it was not upto the prophet (s) to turn people to Islam. They could only relay the Divine message to people.
“And most of the people, although you strive [for it], are not believers.” Quran (Surah Yousuf, 103)
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