An excellent example of his contribution to the history of NT research can be found in his understanding of the impact of Paul's conversion and his opponents on his teaching about justification by faith. In volume three of his work entitled History of the Christian Church, Schaff argues that Paul's
whole theology, doctrinal, ethical, and practical, lies, like a germ, in his conversion; but it was actually developed by a sharp conflict with Judaizing teachers who continued to trust in the law for righteousness and salvation, and thus virtually frustrated the grace of God and made Christ's death unnecessary and fruitless" (302-303; cited in Baird, History of NT Research, 2:49)
I find Schaff's words to be remarkably like those that we read in the Paul and the Law debates of the late twentieth and early twenty-first centuries. His emphasis on the importance of Paul's conversion would be music to the ears of traditional readers of Paul like Seyoon Kim, while Schaff's realization that the "Judaizing teachers" actually brought Paul's theology into development is right in line with many New Perspectivists.
It has been instructive and exciting to watch, retrospectively, the development of these ideas that we all take for granted today!