In Linus or Peter? The Question of Papal Infallibility, Geis argues that the Vatican I proclamation on papal infallibility is directly rendered uncertain, if not outright discredited and disproved, by three passages in Scripture (Mt. 18:18, Lk. 10:1-16, 1 Cor. 10:4). Lexical inquiry into other Scriptural passages dealing with the Scriptural terms "rock" and "cornerstone" solidify this claim. Scrutiny of Patristic sentiment adds to the evidence, as does dating Acts' record of Peter's missionary activities. History shows a tension between wayward papal conduct and dogmatic claims, as well as records of excommunicated popes. 18:18 argues, Geis claims, for the inerrancy of the Spirit's guidance witnessed in the practice and belief of the worldwide Christian community over the millennia. The Roman rite claim for a Divinely ordained Vicariate of authoritative pronouncements, having no Scriptural basis, emerges as an impediment to Christian unity, to Christ prayer "that they may be one" (Jn. 17:20-21).