In a strict sense, any baptized Christian is a child of God and thus a member of his Church (Catechism of the Catholic Church (CCC) 1265-71, 1997). For through baptism we become “partakers of the divine nature” (2 Pet. 1:4). The baptized are members of “a chosen race . . . God’s own people” (1 Pet.2:9; cf. 1 Cor. 12:12ff.; Rom. 12:4-5). And thus each believer has a guardian angel (CCC 336; cf. Mt. 18:10).
In a wider sense, because everyone is made in God’s image and likeness (cf. Gen.1:27-28) and God desires that all human beings be saved (2 Pet. 3:9; Jn. 3:16-17), all human persons can be said to be children of God. And yet we can’t fall into religious indifferentism. On the salvation of non-Catholics, see CCC 846-48).