Karl Barth (pronounced "bart") was a Swiss Reformed theologian whom critics hold to be among the most important Christian thinkers of the 20th century; Pope Pius XII described him as the most important theologian since Thomas Aquinas.Beginning with his experience as a pastor, he rejected his training in the predominant liberal theology typical of 19th-century Protestantism, especially German.

Instead he embarked on a new theological path initially called dialectical theology (due to its stress on the paradoxical nature of divine truth—for instance, God is both grace and judgment), but more accurately called a theology of the Word. Critics have referred to Barth as the father of neo-orthodoxy—a pejorative term emphatically rejected by Barth himself. Barth's theological thought emphasized the sovereignty of God, particularly through his innovative doctrine of election. His theology has been enormously influential throughout Europe and America.