Are Catholics who reject the church’s teachings on the mass, the Trinity, the virgin birth of Christ, abortion, etc. no longer Catholics or are they one for life because of their baptism?
Regarding baptism, the catechism affirms the following: Baptism seals the Christian with the indelible spiritual mark (character) of his belonging to Christ. No sin can erase this mark, even if sin prevents Baptism from bearing the fruits of salvation. Given once for all, Baptism cannot be repeated. (#1272).
That said, your question also seems to touch on the question of how deep one’s communion is with Christ and his Church. And if one were to reject the teachings you mention, they would, in effect, seriously harm, even sever their communion, their unity with the Church. At times the Church must ask those who intentionally dissent to assess their own communion with the Church, and to no longer celebrate a communion that is seriously impaired by refraining from receiving Holy Communion. In rare cases the Church may see a need to formally declare an excommunication exists.
However, even in such cases, given the indelible mark that baptism confers, one can never utterly lose the status of belonging to Christ. By analogy, even if a son or daughter of yours were to wander far from you, live in total contradiction to what you believe, and even curse you to your face and act so badly that you had to erect legal protections for yourself, none of this would change the fact that they are still your son or daughter.
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