The little book is a quick read so long as you are not well-versed enough in Hellenic Greek and scholastic Latin to contest Will's scholarship. The body of the book consists of deconstructing the (not really Pauline) Epistle to the Hebrews -- I think it is fair to summarize that Wills shows this late addition to the Christian canon to have introduced anomalous notions about human and animal sacrifice that have no support in Gospel accounts of Jesus' life. When he's done with Hebrews, Wills gets going on the medieval idea of "substitutionary atonement" -- that God the Father is really a rather nasty demon who had to cause his Son to be tortured to death in order to redeem creation (that's us.) Again, there's no real warrant for this idea in the Jesus story, though it fit well with a feudal and monarchical society and intrudes upon Christians' encounter with Jesus still.
Knock these two props over and the whole theology of the Eucharist as the exclusive gateway to God presided over by a mystically empowered clerical class collapses in Wills' thinking. As he recently put it in an op-ed,
So is Wills still a Christian? Sure. He finds no contradiction between throwing out much of the medieval superstructure of church and continuing to experience the tradition. He affirms the Nicene Creed and other elements of church life he finds enhance his encounter with God, in particular faith that the blessed "body" and "blood" that Christians share in worship recall Jesus' radical practice of eating alongside everyone, even the "impure." And some parts of Church tradition seem to Wills very much worth cleaving to:
Not surprisingly, reviewers have asked Wills whether he is still a Roman Catholic. Though he affirms that all people who name themselves Christians and also whoever seeks God through whatever portal have some window on the divine, he's sticking with the particular tribe of his personal history whether its rulers want him or not: