For a long time Roman-Catholic research into Luther was determined by stereotypes and caricatures that went back to Luther’s opponent, Johannes Cochlaeus. Only in the twentieth century was this approach abandoned, through the work of, among others, Joseph Lortz, Adolf Herte, Erwin Iserloh and Otto Hermann Persch. The last mentioned compared Luther systematically with Thomas Aquinas and thereby laid (along with others) the foundation for an ecumenical approach, which eventually resulted in a joint declaration concerning the doctrine of justification (Gemeinsame Erklärung zur Rechtfertigungslehre) in 1999.