Who’s in? Who’s out? Lumen Gentium 14 answers it:
This Sacred Council wishes to turn its attention firstly to the Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism(Cf. Mk 16, 16; Jn. 3, 5) and thereby affirmed also the necessity of the Church, for through baptism as through a door (people) enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved.
They are fully incorporated in the society of the Church who, possessing the Spirit of Christ accept her entire system and all the means of salvation given to her, and are united with her as part of her visible bodily structure and through her with Christ, who rules her through the Supreme Pontiff and the bishops. The bonds which bind (people) to the Church in a visible way are profession of faith, the sacraments, and ecclesiastical government and communion. (One) is not saved, however, who, though part of the body of the Church, does not persevere in charity. (That one) remains indeed in the bosom of the Church, but, as it were, only in a “bodily” manner and not “in (the) heart.”(Cfr. S. Augustinus, Bapt. c. Donat. V, 28, 39; PL 43, 197: Certe manifestum est, id quod dicitur, in Ecdesia intus et foris, in corde, non in corpore cogitandum. Cfr. ib., III, 19, 26: col. 152; V, 18, 24: col. 189; In Io. Tr. 61, 2: PL 35, 1800, et alibi saepe.) All the Church’s children should remember that their exalted status is to be attributed not to their own merits but to the special grace of Christ. If they fail moreover to respond to that grace in thought, word and deed, not only shall they not be saved but they will be the more severely judged.(Cfr. Lc. 12, 48: Omni autem, cui multum datum est, multum quaeretur ab eo. Cfr. etiam Mt. 5, 19-20; 7, 21-22; 25 41-46; Iac., 2, 14.)
A few comments … This section leaves untouched the notion of those who are incorporated, though in less fully a way. This section also condemns the lack of charity in a believer. A fate worse than unbelievers awaits, according to St Augustine, at any rate.
Catechumens who, moved by the Holy Spirit, seek with explicit intention to be incorporated into the Church are by that very intention joined with her. With love and solicitude Mother Church already embraces them as her own.
And if catechumens, why not others?