Answer by Br. Francis Therese Krautter:
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Not subscribing to precepts does not cause one to become a Protestant. There are five precepts one must follow to be in full communion with the Catholic Church and - as a Catholic - to avoid mortal sin. They are listed here: Catechism of the Catholic Church. Following these precepts is the absolute minimum required to be considered a practicing Catholic. Not following those precepts makes you a “non-practicing Catholic.”
Protestantism is a rejection of specific teachings (or doctrines) coupled with new teachings considered heresy by the Catholic Church.
Believing something that is not taught by the Church, but which is not specifically against the faith is considered heterodoxy.
If the non-adherence to Catholic teaching is willful it is considered formal heresy - if it is simply due to ignorance it is considered material heresy.
Rejection of some teachings of Catholicism makes you a heretic. Rejection of Christianity makes you an apostate. Rejecting the authority of the Catholic Church (while maintaining its teachings) makes you a schismatic. So Protestantism is a schismatic and heretical movement.
Protestantism defines itself not as a “protest” against Catholicism, but as the “profession” of certain fundamental dogmas that are in fact opposed to Catholic dogma. Protestantism also refers to the historical split with the Catholic Church during the Reformation. Being a Protestant therefore requires belonging to a group that has its roots in the Reformation.
Not every non-Catholic (non-Orthodox) Christian is Protestant. Anglicans are not Protestants for example - they did not split as part of the Reformation. Some modern Evangelical Christians - though their doctrines resemble Protestant doctrines - do not trace their roots to a main-line Protestant group.
Unless you join a Protestant group, your heresy does not make you a Protestant.