By our conception in original sin, we are sons (children) of Adam. By our baptism and reception of the state of grace, we become adopted sons (children) of God.
Council of Trent: “By which words, a description of the Justification of the impious is indicated, — as being a translation, from that state wherein man is born a child of the first Adam, to the state of grace, and of the adoption of the sons of God, through the second Adam, Jesus Christ, our Savior. And this translation, since the promulgation of the Gospel, cannot be effected, without the laver of regeneration, or the desire thereof, as it is written: ‘unless a man be born again of water and the Holy Spirit, he cannot enter into the Kingdom of God.’ ” [Sixth Session, Decree on Justification, Chapter IV]
Notice that the “laver of regeneration”, meaning the washing of rebirth in baptism, is what makes us adopted children of God. For the only way that we can enter the state of grace is by some form of baptism. But the Council explicitly states that a baptism of desire (“the desire thereof”) can also make us adopted children of God. For the state of grace, given by some form of baptism, is what makes us sons of God.
Everyone who is in the state of grace, is an adopted child of God. And this includes everyone who loves God, even if they are non-Christian, and everyone who truly and selflessly loves their neighbor, even if they are an atheist or agnostic. For no one can truly love God or neighbor, unless they have the infused theological virtue of love, found only in the state of grace. And everyone who truly loves their neighbor, also loves God, at least implicitly.
The Church has many formal members: baptized Christians. But the Church also has many non-formal members: non-Christians who entered the state of grace by a baptism of desire or of blood. And whenever we see true love, true faith, or true hope among non-Christians, it is evidence of this work of the Holy Spirit, sanctifying persons who are formally outside the Church. The Church is the sole Ark of salvation. But Her membership includes persons who are not formally baptized, and even some who outwardly reject Her. Everyone who is in the state of grace is a member of the Church and a child of God, including many non-Christians.
What should we say about baptized Christians, who are in a state of unrepented actual mortal sin? They are still Christians and still members of the Church, since they retain the indelible character given at baptism and often retain the virtue of faith. But their membership has been harmed by grave sin and the refusal to repent. If they die unrepentant from actual mortal sin, they will suffer forever in Hell. No one in Hell is a member of the Church. Every holy soul in Purgatory and everyone in Heaven is a member of the Church.
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