At the time of the Reformation, the verb ‘protest’ in English primarily meant ‘to affirm, to witness, to testify’ rather than ‘to dissent’ (as we would now consider it). This was also the case for German and other languages which derived cognate words from the Latin protestari (in itself analogous in meaning to the Greek μάρτυρος marturos whence comes the English ‘martyr’).
In the context of the Reformation, Protestant did not primarily mean ‘a person who dissents from the Catholic Church’. Instead it referred to those who affirmed various Reformation doctrines and confessions. More narrowly, it applied to those Germans who affirmed the First Diet of Speyers and rejected the Second.