Here’s the problem: if Christianity teaches unchanging truth how does it adapt to different cultures and different ages?
 
If it does not adapt it ceases to be relevant. If it does nothing but adapt it alters the truths that cannot be changed.
 
The English theologian Blessed John Henry Newman said that for Christianity to be both dogmatic and relevant it required an infallible interpreter. What he meant by this was that Christianity was, by definition a dogmatic religion. That means it holds to certain truths that are anchored in historic facts.  These truths cannot be changed just as the historic facts cannot be changed. The Nicene Creed lays those truths out in the form of theological propositions. Catholic Christians recite those truths every week at Mass and thus affirm that Christianity is a dogmatic religion.
 
These unchanging truths, however, must be applied in real life, and real life is complex, varied and constantly changing. To be real, Christianity must be adapted to different cultures, different ages and different circumstances. The truths cannot be changed, but their application must be. The dogmas cannot be altered, but their presentation and priorities must be flexible.
 
For a religion to be real it must be both dogmatic and practical. It must be unchanging and yet ever changing. For a religion to do both, Newman argues, it must have an infallible interpreter.
 
The infallible religious authority makes the final call and decides what can be changed and adapted, and what cannot. The lack of such an authority causes chaos, division and strife within Protestantism. 
 
“Shall we baptize by immersion, sprinkling or pouring water over the head?”
 
“Shall we have women priests or not?”
 
“Shall we allow divorce and remarriage? Same sex marriage? Artificial contraception? Euthanasia? Abortion?” 
 
The Protestant throws up his hands and asks, “How shall we know? How shall we decide? Shall we take a vote?”
 
Divisions and chaos in church result because of a plethora of questions both small and great, theological, moral, political, economic, cultural, liturgical—you name it.
 
The Protestants have two ways of coping with this conundrum: schism and heresy. The schism solution means when Christians disagree they simply agree to disagree, split up and form yet another new church. The heresy solution is to sacrifice the unchanging truths in some way, and increasingly that way its to dispense with dogma altogether because, “Dogma divides.”
 
The Catholic solution is to have an infallible authority. The catechism teaches that Christ is the infallible authority, and that he grants a measure of his infallibility to his church with the successor of St Peter at her head. 
 
We constantly see the Catholic Church exercising this authority to preserve dogma on the one hand and adapt to changing circumstances on the other. The authority is often exercised through conflict in the church. Catholics quarrel over what can be changed and what cannot. Then they discuss further and finally the referee—in the form of the Pope—makes the final call.
 
A good example of this process was the decision about the ordination of women to the priesthood. The church debated the question. Experts were consulted. Other church bodies were consulted. A decision was made. That decision was strengthened and a conclusion was found. In this case a change could not be made and the final authority explained why. In other cases, as in the rules for determining the validity of marriage changes can be made to adapt to changing times, circumstances and pastoral need.
 
Without this infallible authority Christianity simply doesn’t make sense. Without this authority Christianity drifts into schism or ceases to be dogmatic. 
 
Only an infallible authority can both preserve the faith once delivered to the saints and decide how best to adapt that faith for the salvation of souls in varied circumstances and cultures. With that authority unchanging truths can be applied practically and pastorally.