Philipp Jakob Spener (1635-1705) was a German theologian known as the father of Pietism, a movement within Lutheranism that emphasized personal faith and piety over formalism. He advocated for a more heartfelt and practical approach to Christianity, encouraging believers to engage in Bible study and personal devotion. Spener's influential work, "Pia Desideria," outlined his vision for reforming the church by promoting spiritual renewal and community involvement. His ideas led to the establishment of small groups for prayer and study, which became a hallmark of Pietist practice and significantly impacted the development of modern Protestantism.