Is it forbidden to curse the church with offensive words?

There is no sanctity for a church, and reviling it is not forbidden unless it leads to Christians reviling Islam. Churches are not sacred houses; rather, they are polytheistic houses where companions and children are attributed to Allah. Whoever believes that churches are houses of Allah, or that Allah is worshipped therein, or that what Jews and Christians do is worship of Allah, is a disbeliever.

Shaykh al-Islam Ibn Taymiyyah said that whoever believes in the validity of their religion or helps them establish their religion is a disbeliever. Shaykh Mustafa al-Ruhaibani added that whoever believes that the visit of the People of the Book to their churches is an act of closeness to Allah is an apostate, for denying His (the Almighty's) saying: "Indeed, the religion in the sight of Allah is Islam."

However, if it is feared that reviling churches will lead to Christians reviling the religion of Islam, then it is not permissible to revile them, as a measure to block the means (sadd al-dhara'i'), in accordance with His (the Almighty's) saying: "And do not insult those they invoke other than Allah, lest they insult Allah in enmity without knowledge." Al-Qurtubi said that this ruling remains in effect for the Ummah, so it is not permissible for a Muslim to insult their crosses or churches if the disbeliever is in a position of strength and it is feared that Islam will be insulted.