'I desire mercy, not sacrifice'. Echoing Hosea, Jesus defends his embrace of the unclean in the Gospel of Matthew, seeming to privilege the prophetic call to justice over the Levitical pursuit of purity. And yet, as missional faith communities are well aware, the tensions and conflicts between holiness and mercy are not so easily resolved. In an unprecedented fusion of psychological science and theological scholarship, Richard Beck describes the pernicious (and largely unnoticed) effects of the psychology of purity upon the life and mission of the church.