Why do Christians—especially their clergy—not believe in Islam, despite their good morals and kind dealings, and do these clergy know that Islam is the truth?

The good deeds of a non-Muslim will not benefit them in the Hereafter if they do not believe in Allah and His Messenger, just as was the case with Abu Talib, the Prophet's uncle (peace be upon him). The good character of some non-Muslims might be innate or a means to gain public affection. Muslims must treat such individuals with good character and invite them to Islam, explaining to them the merits of the religion and the importance of ethics within it. Muslims should also apply the teachings of their religion and embody its ethics to be a good example.