What is the ruling on a Muslim being named "Cristiano," which means "Christian," or becoming known by it? And does the one who names him with it commit an act of apostasy? And what is the ruling on not changing this name?

It is not permissible for a Muslim to name himself or others with names that indicate affiliation with a religion other than Islam. Deliberately doing so is considered an act of disbelief (kufr) due to the imitation of disbelievers and the acceptance of their disbelief, as Allah Almighty says: "Indeed, in that case, you would be like them" [An-Nisa 4:140], and as the Prophet, peace and blessings be upon him, said: "Whoever imitates a people is one of them." However, whoever becomes known by such a name without his consent does not commit an act of disbelief, but he must officially change it if possible, or use another name or title by which he is known. It is not permissible to keep this name or to respond to anyone who calls him by it, just as some Companions did when they changed their polytheistic names after embracing Islam. Some scholars have stated that whoever is called "O Jew," or "O Christian," or "O Magian," and responds "Labbayk" (at your service), has committed an act of disbelief.