Emphasis mine in the text:
Those Who Are Not Members Of The Church
Hence there are but three classes of persons excluded from the Church's pale: infidels, heretics and schismatics, and excommunicated persons. Infidels are outside the Church because they never belonged to, and never knew the Church, and were never made partakers of any of her Sacraments. Heretics and schismatics are excluded from the Church, because they have separated from her and belong to her only as deserters belong to the army from which they have deserted. It is not, however, to be denied that they are still subject to the jurisdiction of the Church, inasmuch as they may be called before her tribunals, punished and anathematised. Finally, excommunicated persons are not members of the Church, because they have been cut off by her sentence from the number of her children and belong not to her communion until they repent.
But with regard to the rest, however wicked and evil they may be, it is certain that they still belong to the Church: Of this the faithful are frequently to be reminded, in order to be convinced that, were even the lives of her ministers debased by crime, they are still within the Church, and therefore lose nothing of their power.
The question of who is in the Church is not a simple answer, because there are those who are, for the sake of some distinctions, not members of the Church:
12 Q. The many societies of persons who are baptised but who do not acknowledge the Roman Pontiff as their Head do not, then, belong to the Church of Jesus Christ?
A. No, those who do not acknowledge the Roman Pontiff as their Head do not belong to the Church of Jesus Christ.
24 Q. To be saved, is it enough to be any sort of member of the Catholic Church?
A. No, to be saved it is not enough to be any sort of member of the Catholic Church; it is necessary to be a living member.
25 Q. Who are the living members of the Church?
A. The living members of the Church are the just, and the just alone, that is, those who are actually in the grace of God.
26 Q. And who are the dead members?
A. The dead members of the Church are the faithful in mortal sin.
30 Q. Suppose that a man is a member of the Catholic Church, but does not put her teaching into practice, will he be saved?
A. He who is a member of the Catholic Church and does not put her teaching into practice is a dead member, and hence will not be saved; for towards the salvation of an adult not only Baptism and faith are required, but, furthermore, works in keeping with faith.
The fruit of all the Sacraments is common to all the faithful, and these Sacraments, particularly Baptism, the door, as it were, by which we are admitted into the Church
Baptism is the "door" to the Church, and one may, in some cases, refer to the Baptized as members of the Church, although, those in mortal sin are spiritually dead members, and those who are heretics, schismatics, excommunicated are not considered members, although, still subject to the authority of the Church.
In this sense, one can easily qualify who exactly are "members" of the Church as being near, imperfect, dead, etc.
When considering who is a member of the Church, broadly speaking, it is important to keep the Church's doctrine in mind at all times.
Declaring a person who was not judged to be a heretic by the Church to be one, and then claiming they are not members of the Church, leaves one open to the same accusations being made, especially if they actually deny clear statements in the Catechism in practice.
Accepting the words is not enough: we need the acts.
That is why adults who are baptized can be outside the Church. They have acted against the Church and excluded themselves as judged by the Church.
As a reminder, the sublime statement of the Catechism is that heretics, schismatics, and the excommunicate are
beyond the Church's pale and not considered members of the Church because of it, but one could easily make the argument that they still belong to the Church, unlike infidels who never did.
