According to chapter 4 of “Pastor Aetemus” (the text on papal infallibility promulgated by Vatican I on July 18th, 1870), it is “that infallibility with which the divine Redeemer willed that his Church should be endowed in defining doctrine regarding faith or morals” that may, in certain circumstances, enable the bishop of Rome, in his capacity as successor of St Peter, to define a doctrine as to be held by the universal Church. What the Church has as a permanent endowment, built in structurally so to speak (instructam in the original Latin), the successor of St Peter may on occasion have available to him, enabling or “powering” him (pollere). The problem of interpretation here lies not only in discerning in what sort of circumstances the bishop of Rome may thus be “enabled”, but in discovering the sense in which the Church herself is endowed with infallibility at all. Problematic as discerning a papal decision that enjoys infallibility may be, it is nothing to exploring the mystery of the Church’s infallibility.

It is a negative term, like so many others in theology. To be infallible is to be undeceiving and undeceivable, to possess immunity from either leading or being led into error. Christ and the Holy Spirit can neither deceive nor be deceived, and since the Church is, mystically and sacramentally, the body of Christ and the continuation of Pentecost, the Church is infallible. This is the faith shared by all Christians, Catholic and Orthodox.