Most people are not aware that changing or transferring from one religion to another is a sin or not. Some would say that there is nothing wrong in transferring from one church to another as long as you keep your personal relationship with Jesus Christ. With this ignorance of a most basic tenet of the Christian faith various cults and Protestants denominations have lured so many souls away from Jesus Christ and His Catholic Church. In order to satisfactorily answer the question Is changing religion a sin? We must first define what religion and sin is. The Catholic Dictionary (1909) defined religion in two dimensions;
Objectively, it consists in doctrines and precepts by which man seeks to bring about this union. Religion is true when its doctrines and precepts are either dictated by right reason or revealed by God; if the former, it is called natural religion, if the latter, supernatural religion. Religion is false if, when claiming to be revealed, it is unable to show a divine guarantee, or when its dogmas and practices sin against right reason and conscience.
Subjectively, religion is the attitude of the man who rules his thoughts, words, and actions according to right reason and revelation. In this latter sense religion is a special virtue allied to justice, because it prompts man to render to God what is due Him by strict right from His rational creatures. As such, religion is a strict obligation incumbent on every man. It is also the means by which man is to work out his final destiny.
The objective dimension of religion is defined as the system of belief it refers to doctrine and dogma that are divinely revealed by God. Judging by the objectivity of religion any system of belief that is not divinely revealed by God is utterly false. Subjectivity of religion is the response of man towards God, it refers to man’s eternal obligation to adore God. Henceforth, if a person cannot fulfill his obligation to God by worshiping Him in the Holy Eucharist he is committing a grave mortal sin.
Another point of consideration is the nature of Sin the Catechism of the Catholic Church defined sin as;
Sin is an offense against reason, truth and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as “an utterance, a deed or a desire contrary to the eternal law. CCC 1849
Sins can be distinguished according to their objects, as can every human act; or according to the virtues they oppose, by excess or defect; or according to the commandments they violate. They can also be classed according to whether they concern God, neighbor, or oneself; they can be divided into spiritual and carnal sins, or again as sins in thought, word, deed, or omission. The root of sin is in the heart of man, in his free will, according to the teaching of the Lord: “For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man.” But in the heart also resides charity, the source of the good and pure works, which sin wounds. CCC 1853
Sin as defined in the Catechism is a transgression against God, it is an offense against reason and the eternal law. When a person who belongs to the true Church (Catholic Church) changes his religion or transferred to another christian church he is committing a grave sin because it is an offense against reason and God’s eternal law. Knowing that the Catholic Church is the true Church yet he still chooses to leave the church for another is an offense against reason. It is also an offense against God’s eternal law because God’s will for us is to belong into the body of Christ (Col.1:18). The sin of leaving the true Church for another is called apostasy;
Incredulity is the neglect of revealed truth or the willful refusal to assent to it. “Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him.” CCC 2089
When a Catholic abandoned the Catholic faith and becomes a Muslim, Jew, Pagan and any other religion that is not Christianity he is guilty of apostasy. But when he denied a single doctrine and dogma of the Church whether he transferred to a Protestant Church or not he is guilty of heresy. Whether it is apostasy or heresy as long as it is committed with the knowledge of the person he is guilty of mortal sin.
Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter it or to remain in it, could not be saved. Lumen Gentium No.14
Therefore, a Catholic who knows that the Catholic Church is the true Church yet he refuses to remain in it cannot be saved. There is no justifiable reason for a Catholic to transfer to a Protestant Church. Now, going back to the question Is changing religion a sin? Taking in consideration the teaching of the Church, changing of religion or transferring to Protestant Churches is a grave mortal sin. When a Catholic transfers to a Protestant Church or abandoned the Christian faith he commits an offense against reason and God’s eternal law. Catholics should have studied or learned more about their Catholic faith rather than abandoning it and joining other Protestant denominations. Anyone who abandons the Catholic faith he can no longer partake in the Holy Eucharist which fulfills man’s obligation towards God, worship in a Protestant Church do not fulfill this obligation. But when a Protestant or a non-Christian converted to Catholicism he made a decision which is commendable and pleasing to God.