Peace, mercy, and blessings of God be upon you. Is backbiting Christians permissible?

Backbiting is forbidden in the case of a Muslim and a Dhimmi among the disbelievers, due to the saying of the Prophet, peace and blessings be upon him: "Whoever causes a Jew or a Christian to hear (something bad about himself) will be in the Fire." As for the combatant, backbiting him is permissible, provided that the backbiter says nothing but the truth.