Is it sin to change from a Catholic to a Christian?

Answer 1

Roman Catholic Answer

First of all, one can not change from Catholic to Christian because a Catholic is a Christian. But, if by Christian you are referring to a "protestant" then yes, it is a sin punished by excommunication; and, if unrepented before death, would involve one's damnation. This is called formal heresy (as opposed to material heresy) and is very serious.

from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980

Heresy. Commonly refers to a doctrinal belief held in opposition to the recognized standards of an establish system of thought. Theologically it means an opinion at variance with the authorized teachings of any church, notably the Christian, and especially when this promotes separation from the main body of faithful believers.

In the Roman Catholic Church, heresy has a very specific meaning. Anyone who, after receiving baptism, while remaining nominally a Christian, pertinaciously denies or doubts any of the truths that must be believed with divine and Catholic faith is considered a heretic. Accordingly four elements must be verified to constitute formal heresy; previous valid baptism, which need not have been in the Catholic Church; external profession of still being a Christian, otherwise a person becomes an apostate; outright denial or positive doubt regarding a truth that the Catholic Church has actually proposed as revealed by God; and the disbelief must be morally culpable, where a nominal Christian refuses to accept what he knows is a doctrinal imperative.

Objectively, therefore, to become a heretic in the strict canonical sense and be excommunicated from the faithful, one must deny or question a truth that is taught not merely on the authority of the Church but on the word of God revealed in the Scriptures or sacred tradition. Subjectively a person must recognize his obligation to believe. If he acts in good faith, as with most persons brought up in non-Catholic surroundings, the heresy is only material and implies neither guilt nor sin against faith. (Etym. Latin haeresis, from the Greek hairesis, a taking, choice, sect, heresy.)

Answer 2

A:Both yes and no. The religion you left will say it is a sin, while the religion you join will say it is good for your soul. If you convert from Christianity to Islam, the church will say you have committed a grievous sin, while Islam will say you have been saved. If you convert from Islam to Christianity, Muslims will call you an apostate and reserve the very depths of hell for you in the afterlife, but your church will say you have been saved and will go to heaven.

So it balances out - unless you become an atheist.

Minor Note:

The Human Rights Act gives you the right to choose your own religion and beliefs without discrimination. Discriminating someone because they switched religions or beliefs is against the law.