Which Persons are Members of the One True Church?
1. The Three Persons of the Trinity, including Jesus with his human nature
2. The Blessed Virgin Mary
3. All the souls in Heaven
4. All the souls in Purgatory
5. All the holy angels
6. baptized Catholic Christians in a state of grace
7. baptized non-Catholic Christians in a state of grace
8. non-Christian believers in a state of grace by a baptism of desire or blood
9. atheists or agnostics in a state of grace by a baptism of desire or blood
10. baptized Christians in a state of unrepented mortal sin, who have retained the virtue of faith (though their membership is, we might say, partial or endangered until they repent)
11. non-Christians who have departed from the state of grace by mortal sin, if they retain the virtue of faith (see #10)
12. Christians who have separated themselves from the Church by apostasy, heresy, or schism, if they retain the state of grace by a sincere but mistaken conscience.
13. atheists and agnostics who obtain and retain the state of grace by the love of neighbor (which always includes the love of God, at least implicitly)
14. prenatals, infants, and young children, unbaptized, who die at that young age, because they receive a baptism of blood prior to death
15. prenatals, infants, and young children, unbaptized, who are innocent of sin and therefore potential members of the Church (they are like persons standing at the threshold of the house).
Which persons are not members of the Church?
1. the fallen angels
2. the souls in Hell, including the limbo (fringe) of Hell
3. Christians who have committed actual mortal sin and also lost the virtue of faith by apostasy, heresy, or schism in bad conscience.
4. non-Christians who have culpably (to the extent of actual mortal sin) failed to enter the state of grace by some form of baptism
5. non-Christians who have never entered the state of grace and have committed any actual mortal sin without repentance
6. non-Christians who have departed from the state of grace by actual mortal sin and also by the loss of the virtue of faith
The Council of Trent taught two doctrines of interest to this topic:
CANON XXVIII. If any one says, that, grace being lost through sin, faith also is always lost with it; or, that the faith which remains, though it be not a lively faith, is not a true faith; or, that he, who has faith without charity, is not a Christian; let him be anathema.
For faith, unless hope and charity be added thereto, neither unites man perfectly with Christ, nor makes him a living member of His body.
So if a Christian loses the state of grace by an actual mortal sin, but retains the virtue of faith, he is still a Christian and therefore in some sense a member of the body of Christ, but he is not a living member, since his faith is not enlivened by charity (love) and hope. By this doctrine, we should understand that there are different degrees or types of membership. Full membership in this life is found in the baptized believing and practicing Catholic, who retains the state of grace.
Christians who do not have access to the Sacrament of Reconciliation and non-Christian believers, who have departed from the state of grace by an actual mortal sin, can return to the state of grace by an act of perfect contrition (sorrow for sin out of love of God and neighbor).
Atheists and agnostics who have departed from the state of grace by an actual mortal sin can return to the state of grace by an act of implicit perfect contrition (sorrow for sin out of love of neighbor).
Everyone in a state of grace is a member of the one true Church, at least implicitly. But apostates, heretics, and schismatics are no longer formal members of the Church. Everyone who commits an actual mortal sin, and who thereby loses the state of grace, but still retains the virtue of faith (because the type of sin did not destroy that virtue), remains in some sense a member of the Church, a member cut off by the lack of love, yet tenuously joined by the continuation of faith.
Some persons are formal members of the Church, and other persons are non-formal members (mystical members). The Church is very broad and includes many non-Catholics and non-Christians and even some unbelievers. All persons of good will are members of the Church.
Please take a look at this list of my books and booklets, and see if any topic interests you.