In his first audience on TOB, Pope John Paul focused on 2 things. First, he was then in preparation for the upcoming Synod of Bishops on "The role of the Christian Family", his second focus was a phrase "from the beginning."
The synod he referred to in the audience actually took place in 1980 for 5 weeks with 200 bishops present to discuss the Christian Family in the modern world. It significantly influenced his work. One year after the synod, the Pope issued "On the Family", the lengthiest treatise on marriage and family ever created in the Catholic Church.
But, as to "from the beginning", the Pope said a lot in a short span. Catholics believe in the immutability of the Bible, but also that it must be put into its proper context, not left naked, so as the Pope talks about Jesus speaking on marriage with the Pharisees, he looks for the nuance, the hidden meaning, and the context.
The biblical discussion focused on this section of the Bible:
"And Pharisees came up to him and tested him by asking, 'Is it lawful to divorce one's wife for any cause?' He answered, 'Have you not read that he who made them from the beginning made them male and female, and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So they are no longer two but one flesh. What therefore God has joined together, let not man put asunder.' They said to him, 'Why then did Moses command one to give a certificate of divorce, and to put her away?' He said to them, 'For your hardness of heart Moses allowed you to divorce your wives, but from the beginning it was not so'" (Mt 19:3ff., cf. also Mk 10:2ff.).
The Pharisees typically were haggling over legalistic points, where Jesus was talking about natural law, and the desires of the Father, made known in part in His Word, which at the time was actually standing in front of them - Word Made Flesh.
But Jesus referred the "beginning" referred to biblically in Genesis. As the Pope says:
Christ quoted Genesis 1:27 in summary form: "In the beginning the Creator made them male and female." The original passage reads textually as follows: "God created man in his own image; in the image of God he created him; male and female he created them." Subsequently, the Master referred to Genesis 2:24: "Therefore, a man leaves his father and his mother and cleaves to his wife, and they become one flesh."
What Jesus says in the Gospel account makes the words of Genesis, no longer only de facto, but also normative, or normal, as in natural law for human beings, and he does so by making reference to the most primary of all scriptures, the early part of the book of Genesis.
As the Pope concludes on this topic:
However, that significant expression "from the beginning," repeated twice, clearly induced his interlocutors to reflect on the way in which man was formed in the mystery of creation, precisely as "male and female," in order to understand correctly the normative sense of the words of Genesis. This is no less valid for the people of today than for those of that time. Therefore, in the present study, considering all this, we must put ourselves precisely in the position of Christ's interlocutors today.
The Pope uses a technique that good homilists use, inviting the listener or reader to put themselves into the biblical narrative, in this case to see through the eyes of our own Pharisaical judgments what the Master is telling us all individually.