The quick answer is the Holy Spirit. It is God, of course, who is the source and end of everything.
But insofar as orthodoxy is a humanly-mediated affair, it is the official teaching office of the Church, the Magisterium, who has been entrusted the power to define, proclaim and defend the truths of our faith by Christ himself, who said to Peter, the first recipient: “Whatever you bind on earth will be bound in heaven, and whatever you loose on earth will be loosed in heaven.” (Mt 18,18)
These are words that have to be taken by faith, otherwise there is no way we can make sense of them, given the limitations of Peter and the men who succeed him to whom these words can be addressed through the ages.
No matter how brilliant these men are, they just cannot be correct all the time. It is rather this divine guarantee that should make us feel assured of the orthodoxy of the doctrine of our faith as defined by the Magisterium, the Church’s official teaching office.
This point has to be kept in mind as we go through this messy and complicated issue of the Amazonian Synod that has lately stirred ecclesiastical circles intensely. In it, Pope Francis is launching a very bold move to question some current ways of understanding what orthodoxy is.
For one, orthodoxy does not mean maintaining the status quo. There certainly are things that need to change given the changes of circumstances. These are things that can and should change.
Of course, to identify which things as currently understood and held can change may not be easy to do. And that is why the Pope is asking that those in the Synod, which is a collegial way of figuring out what the Holy Spirit is telling the Church today, to speak in ‘parrhesia,’ that is to say, to speak candidly.
That, of course, does not mean that what one says in parrhesia is correct. But everyone should feel free to speak what is in his mind and heart openly. In that way, things are put into the open and can be sorted out, in a manner of speaking, in a collegial way, which is the best way to discern what the Holy Spirit is telling us.
Of course, the collegial way means the Pope always with all those who comprise the college. It is the Pope, being the successor of Peter, who has the authority to bind and to loose that was first given to Peter.
There are many issues to be taken up in this Amazonian Synod, and one of them is the possibility of ordaining married men to the priesthood to fill the dire need for priests in that region of the world. This brings to the fore the question of whether there can be exception to the priestly celibacy rule.
Obviously, such exception can be done. It already has been done to some married Anglican pastors who converted to Catholicism. Of course, these exceptions are given with clear conditions.
As to the question that giving these exceptions would usually lead to abuses, that is another matter that has to be resolved accordingly. But we have to realistic. We have been abusing God’s laws all the time, since Adam and Eve. We just cannot negate giving exceptions to some rule due to the possible abuses. We are not in a perfect world.
The priestly celibacy rule is a disciplinary and not a dogmatic thing. And as such, some exceptions can be made. It is different when we talk about ordination of women which is already a dogmatic doctrine. No exception can be made here.
In any event, as the Pope has begged, we just have to pray a lot so that this Synod would truly reflect what the Holy Spirit is telling us regarding the Church in the Amazon today.