Are Christians considered infidels, and what is the ruling on dealing with them with good manners and sincerity?

Whoever did not embrace Islam after the prophethood of Prophet Muhammad (peace be upon him) is a disbeliever. It is permissible to treat the People of the Book with good manners, provided they are not belligerent, as Allah Almighty says: (Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes - from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly.) However, it is not permissible to show them affection, companionship, or sincere loyalty, as Allah Almighty says: (O you who have believed, do not take the Jews and the Christians as allies. Some of them are allies of one another. And whoever of you takes them as allies - indeed, he is one of them. Indeed, Allah guides not the wrongdoing people) and His saying: (O you who have believed, do not take as intimates those other than yourselves, for they will not spare you [any] ruin. They wish what would harm you. Hatred has already appeared from their mouths, and what their breasts conceal is greater. We have certainly made clear to you the signs, if you will use reason.)