Is it permissible to call Christians as Kafirs (disbelievers)?
Yes, it is permissible to call and describe Jews and Christians (Part No. 2; Page No. 144) as Kafirs. We can do this as Allah Himself does so, as He (Exalted be He) says:
Those who disbelieve from among the people of the Scripture (Jews and Christians) and Al-Mushrikûn, were not going to leave (their disbelief) until there came to them clear evidence.
The People of the Book are
the Jews and
the Christians. He also says:
Surely, in disbelief are they who say that Allâh is the Messiah, son of Maryam (Mary).
And Allah says:
Surely, disbelievers are those who said: “Allâh is the third of the three (in a Trinity).”
In addition to the other Ayahs (Qur'anic verses) and Hadith that refer to their Kufr (disbelief).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.