Protestant Christianity began in the early 16th century as a reform movement directed against Roman Catholic doctrines and practices. Early leaders such as Martin Luther, Ulrich Zwingli, and John Calvin laid out the movement's foundational principles, including the conviction that salvation is by god's grace alone, that the Bible is the sole authority of faith and practice, and that the church is a "priesthood of all believers." In the 19th and 20th centuries, Protestant Christianity spread world-wide. Ecumenical efforts have brought many groups into close working relationships and produced unions of churches, though disagreements continue. The term Protestantism thus has become a broad umbrella for a variety of beliefs and institutions that retain some connection with the past as they express renewed forms of religious vitality in the present.