What is the ruling on eating the meat of an animal slaughtered by a Muslim who is not observant of his religion's teachings, and that of an animal slaughtered by a Christian or Jew who is not observant? And which is better: the animal slaughtered by the non-observant Muslim, or the animal slaughtered by the observant Christian or Jew?

The basic ruling is that the slaughtered animals of the People of the Book are permissible, unless it is known that they were slaughtered in a manner other than the prescribed Islamic way, or if they were dedicated to other than Allah. The fact that a person from the People of the Book is not devout in their religion does not affect the permissibility of their slaughtered animal. As for the slaughtered animal of a non-practicing Muslim: if their non-compliance amounts to disbelief after the establishment of proof against them, then their slaughtered animal is not permissible to eat. If their non-compliance does not reach the level of disbelief, then their slaughtered animal is permissible to eat. One who abandons prayer completely is a disbeliever, while one who prays sometimes and abandons it sometimes is a transgressor (fasiq). One who abandons fasting does not disbelieve. All of this applies to those who do not deny the obligation itself.