Strictly speaking, one who makes protest of his belief in or against something; in general usage a member of one of the sects which arose in Western Europe at the time of the Reformation or of one of the offshoots of these at a later time. The term originated at the Diet of Speyer in April 1529, when it affirmed that where the "new religion" had been established it might be tolerated, but that its adherents should not interfere with the Mass or with the attendance of Catholics thereat. Against this decree permitting Catholic worship a number of the members, led by the Elector Frederick of Saxony, made a solemn "protest," hence the name as applied to those who opposed Catholicism. Lutherans, Reformed, and later Anglicans were all content to be known by this name, and with the rise of other sects it was accepted by them, so that its sole meaning became that of opposition to Rome. In general Protestants have accepted as fundamental three principles:

the supremacy of the Scriptures
justification by faith alone
the priesthood of all believers

Even in this they are not fully agreed, and many, Anglicans especially, who would not accede to this as a statement of their belief, called their religion Protestant. With the rise of the pro-Roman party in Anglicanism, both the term and the thing are repudiated. However, in general all religious bodies separated from the Catholic Church and not comprehended in one of the schismatic Eastern Churches, are classed as Protestants.

Bibliography Information
Entry for 'Protestant'. 1910 New Catholic Dictionary. https://www.studylight.org/dictionaries/eng/ncd/p/protestant.html. 1910.

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