One can seperate onself from the church and therefore from the body of Christ through heresy or schism or apostasy, this much is clear from the constant teaching of the church.
HOWEVER this is also a side issue, the real issue is that whilst individual Orthodox OR protestants OR even by some mystery non-christians can belong to the body of christ, the body corportate i.e their church, sect or religon, does not and cannot be said to aid in their salvation. ‘Error is an obstacle to the Holy Spirit’
Surely, as stated by Pope Paul VI in
Unitatis Redintegratio (1964) the separated Churches are aided in their salvation through the separated Churches, because “the Spirit of Christ has not refrained from using them as means of salvation” :
The brethren divided from us also use many liturgical actions of the Christian religion. These most certainly can truly engender a life of grace in ways that vary according to the condition of each Church or Community. These liturgical actions must be regarded as capable of giving access to the community of salvation.
It follows that the separated Churches(23) and Communities as such, though we believe them to be deficient in some respects, have been by no means deprived of significance and importance in the mystery of salvation. For the Spirit of Christ has not refrained from using them as means of salvation which derive their efficacy from the very fullness of grace and truth entrusted to the Church.
…
The daily Christian life of these brethren is nourished by their faith in Christ and strengthened by the grace of Baptism and by hearing the word of God. This shows itself in their private prayer, their meditation on the Bible, in their Christian family life, and in the worship of a community gathered together to praise God. Moreover, their form of worship sometimes displays notable features of the liturgy which they shared with us of old.
- Cf. CONC. LATERANENSE IV (1215) Constitutio IV: Mansi 22, 990; CONC. LUGDUNENSE II (1274), Professio fidei Michaelis Palaeologi: Mansi 24, 71 E; CONC. FLORENTINUM, Sess. VI (1439), Definitio Laetentur caeli: Mansi 31, 1026 E.
vatican.va/archive/hist_councils/ii_vatican_council/documents/vat-ii_decree_19641121_unitatis-redintegratio_en.html
If one is a Catholic to begin with, and adopts heresy or schism or apostacy, then they have separated
due to mortal sin, as explained in the Baltimore Catechism, they are no longer fully incorporated. One can be in objective heresy, or schism, or apostacy, in the state of invincible ignorance, yet not in mortal sin due to it.
58. In the face of new problems and situations and of an exclusive interpretation of the adage: “
salus extra ecclesiam non est”,
[88] the magisterium, in recent times, has articulated a more nuanced understanding as to the manner in which a saving relationship with the Church can be realized. The Allocution of Pope Pius IX,
Singulari Quadam (1854) clearly states the issues involved: “It must, of course, be held as a matter of faith that outside the apostolic Roman Church no one can be saved, that the Church is the only ark of salvation, and that whoever does not enter it, will perish in the flood. On the other hand, it must likewise be held as certain that those who live in ignorance of the true religion, if such ignorance be invincible, are not subject to any guilt in this matter before the eyes of the Lord”.
[89]
[88] Cf. BONIFACE VIII, Bull “Unam Sanctam”: “Porro subesse Romano Pontifici omni humanae creaturae declaramus, dicimus, diffinimus omnino esse de necessitate salutis”,
DS 875; cp.
DS 1351;
ND 875: “Furthermore we declare, state and define that it is absolutely necessary for the salvation of all human beings that they submit to the Roman Pontiff”.
[89] Pius IX, Allocution “Singulari Quadam”, DS 2865i;
ND 813.
vatican.va/roman_curia/congregations/cfaith/cti_documents/rc_con_cfaith_doc_20070419_un-baptised-infants_en.html