Protestantism is a branch of Christianity that follows the theological tenets of the Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century against errors, abuses, and discrepancies.

Protestantism emphasizes the Christian believer's justification by God in faith alone (sola fide) rather than by a combination of faith with good works as in Catholicism; the teaching that salvation comes by divine grace or "unmerited favor" only (sola gratia); the priesthood of all believers in the Church; and the sola scriptura ("scripture alone") that posits the Bible as the sole infallible source of authority for Christian faith and practice. Protestants reject the Catholic doctrine of papal supremacy, and have variant views on the number of sacraments, the real presence of Christ in the Eucharist, and matters of ecclesiastical polity and apostolic succession. Other Protestant denominations and non-denominational Protestants may be typically unconcerned about most of these theological issues and focus only on their perception of explicit Christian teachings in the Bible itself. The five solae of Lutheran and Reformed Christianity summarize basic theological differences in opposition to the Catholic Church. Today, it is the second-largest form of Christianity, with a total of 800 million to 1 billion adherents worldwide or about 37% of all Christians.
What is Protestantism and when did it start?
Ali Malekshahi