Two recent books share a theme that might be called, “What hath Protestantism wrought?” Professor Brad Gregory of Notre Dame University, in The Unintended Reformation, tracks down the numerous historical developments by which Protestantism, initially intended to reform the Church, gradually resulted in thousands of “denominations” sending contradictory messages, but fenced off from mutual conflict and influence by relativistic and tolerant secularistic mechanisms. Devin Rose, in If Protestantism Is True, recaps a train of exhaustive personal theological investigations that led him unexpectedly into the arms of Catholicism.

The seemingly inspired idea of the first Protestant “reformers,” in view of the perceived corruption and mismanagement of...