"I appreciate the Holy Father's point that the document has to be read as a whole, and particularly his insistence that it be read in order and in its entirety for its meaning to be understood," he said.
"I think part of the reason Amoris laetitia has been considered 'controversial' by some people is precisely because there have been misguided attempts to read chapter 8, or even specific lines of chapter 8, in isolation and outside of the essential context of the preceding chapters. The necessary context of the wider document clearly does situate Amoris within the Church's traditional teachings."
The pope said that, in treating "ethical situations," the exhortation "follows the classical doctrine of St. Thomas Aquinas," the thirteenth century theologian whose work is commended specifically by the Church for the formation of priests and theologians.
Petri told CNA that the pope's invocation of Aquinas made sense.
"Regarding the pope's description of the exhortation as within the classical tradition of Thomistic thought – if the document is read properly and according to the pope's own instruction then yes it is," Petri said.
"Chapter 8 itself at one point references the Summa and Aquinas' observation that the more detailed and complicated a situation is, the less general norms and teaching can seem to be applicable, and this is true.
"Of course, it is important to understand that, for Aquinas and for the Church, this applies to human law and even natural law, to a degree, but certainly not to the words of the Lord in the Gospel – that is a completely different matter."
The pope's letter quoted St. Vincent of Lerins, a fifth century monk, on the development of Church teaching, citing him in Latin: "ut annis scilicet consolidetur, dilatetur tempore, sublimetur aetate."
Dr. Jacob Wood translated this for CNA.
"What St. Vincent says is that doctrine 'is solidified over the years, extended with time, and refined with age,' and the pope has actually invoked this quote on a number of different occasions. He laid out for us how he understands it on May 8, 2017, when he referenced these words and said that true doctrinal development 'is the same truth, but it helps us understand it better.'"
"I think the pope's invocation of this particular quote, which he has already explained his understanding of, tells us a lot about how he wants the Church to understand his teaching in Amoris laetitia," Wood told CNA.
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Fr. Petri agreed, saying that the exhortation's controversial passages should be read as "part of a coherent and complimentary whole." But he echoed Francis' warning against reading some parts in isolation.
"If you read the whole document, as the pope is inviting people to do, as a call for people to move and live in grace, its meaning is clear. If people choose to read specific lines or footnotes out of context and try to apply Thomistic thought to imply that the instructions of Christ must be somehow mitigated or considered inapplicable, that would be completely alien to St. Thomas."
Dr. Wood agreed, noting that such an interpretation would have been equally foreign to St. Vincent.
"St. Vincent lived at the beginning of the fifth century, when the Church was going through the major Christological and trinitarian controversies as it sought to articulate true doctrine more clearly and, at the same time, refute heresies. You can imagine how this question of doctrinal development was central to St. Vincent's time – right between the councils of Ephesus and Chalcedon. Each council articulated important doctrines a little better, a little clearer, but always in continuity with what had come before and – of course – in absolute harmony with the divine teachings received from Christ himself."
In his letter to Walford, Pope Francis said that, while not proposing a handbook for dealing with individual circumstances, it did address "current and concrete problems are dealt with: the family in today's world, the education of children, marriage preparation, families in difficulty, and so on" but that these were treated with the "magisterial hermeneutic of the Church."
Fr. Petri said that Amoris laetitia's message of love and support for couples in difficult situations answered a real pastoral need, one that should not be obscured by attempts to make it say something it does not.