When considering the traits of an infallible authority there are three other sets of paired qualities that balance one another, and which are necessary.

First, the Church that speaks with authority must be invisible. In other words, there must be a quality to it which is spiritual and God given and greater than any visible boundaries. This invisible quality means, for instance, that the true members of the church are not defined and specified. The members of this church and the truth she teaches must be ‘known to God alone.’
However, a Church that is only invisible is not satisfactory. We also have to be able to identify this church in history and at the present time. This is why the Catholic church teaches that Christ’s church ‘subsists’ within the Catholic Church. If you want to find Christ’s true church, it is the Catholic Church, but we realize that the true Church is also greater than the visible church.
The authority we are looking for also has to both human and divine. If it is to be infallible, then the church, as an institution, must be divinely inspired and founded by divine action, for it cannot be infallible if it is merely a human institution. However, for this authority to be real and to be active in the world, it must also be incarnated in a human institution. 
The final pair of qualities, which goes along with the divine inspiration of the Church, is that this infallible authority must be grounded and fully integrated with Holy Scripture, and the divine inspiration of the Church itself must be discovered in Scripture. If anything is important for man’s salvation (and therefore necessary to be revealed in Scripture) then it is this: for without this foundation the rest of revelation remains subject to private interpretation.
On the other hand, this authority must not be founded only on Scripture as some sort of blind commitment to an arbitrary religious text. The authority must also be congruent with human reason and compatible with the rest of a reasonable perception of reality. The authority grounded in, and stated in Scripture must also make sense, and the understanding of Scripture itself and its inspiration must be credible and consistent with human reason. If this pair of traits is applicable to the authority itself, then they must also be applicable to the teaching which that authority promulgates. In other words, the what that authority teaches must also be congruent with Scripture and human reason.
When we examine these traits we discover that there is really only one church that has the audacity to claim them all. What other church claims that it’s foundation is by God in Christ himself? None. All others claim, at best, that they share in the apostolic foundation by virtue of apostolic succession, but none claim to be the one church that is founded by Christ on the apostle Peter, and continued to the present day.
It is necessary for every Christian, therefore, to confront these claims and make of them what he will. The old knock down argument can be applied: either Catholics are bad, mad or their claims for this authority are true. That is to say, the Catholic Church, which makes such a claim must be bad for we are lying and we know it, and we wish to deceive as many as possible, or we are mad: we believe something that cannot be true, and even if we are honestly and sincerely mistaken, this too is a form of madness, for we live our lives committed to an unreal proposition.
The other option is that the historic claims for the authority of the Catholic Church are what they claim to be, and as such, the Catholic Church fulfills all the traits we would look for in an infallible authority.
What this means and how this infallible authority is exercised will be the subject of tomorrow’s post.