How do we respond to those who are not convinced that Christians are disbelievers (kafirun)?

It is regrettable that some Muslims are unaware that whoever adopts a religion other than Islam is an infidel destined for eternal Hellfire, and whoever denies this is in grave danger. How can one doubt the disbelief of someone who curses Allah, or describes Him with imperfections, or claims that the Prophet (peace be upon him) was a liar?! Scholars have explicitly stated that whoever doubts the disbelief of a disbeliever whose disbelief is known by necessity, such as Jews and Christians, is himself a disbeliever.

The response to this should be by reciting the explicit verses regarding the disbelief of Christians, such as His (Allah's) saying: ﴿Certainly, disbelievers are those who say, "Allah is the Messiah, son of Mary"﴾ [Al-Ma'idah:17], and His saying: ﴿Certainly, disbelievers are those who say, "Allah is the third of three"﴾ [Al-Ma'idah:73], and other verses and Hadiths that indicate the disbelief of the People of the Book and their eternal dwelling in Hellfire, and affirm that whoever receives the message of the Prophet (peace be upon him) and does not believe in him is a disbeliever.