St. John Paul II, in Ut Unum Sint § 37, quoted Unitatis Redintegratio, §11:
When comparing doctrines with one another, they should remember that in Catholic doctrine there exists a ‘hierarchy’ of truths, since they vary in their relation to the fundamental Christian faith. Thus the way will be opened by which through fraternal rivalry all will be stirred to a deeper understanding and a clearer presentation of the unfathomable riches of Christ.
This does not mean that the “faith which was once for all delivered to the saints” Jude 3 can be divided into some parts that are important and must be upheld while others can readily be discarded at our convenience. Rather, it means that the different truths of our faith are organized around a center:
§ 234 The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them. It is the most fundamental and essential teaching in the hierarchy of the truths of faith. The whole history of salvation is identical with the history of the way and the means by which the one true God, Father, Son and Holy Spirit, reveals himself to men and reconciles and unites with himself those who turn away from sin.
At ground level, Rabbi Yeshua told us to feed the hungry, welcome strangers, clothe the naked, visit the sick, and visit those in prison Mt 25:35. A woman with a husband to take care of needs to prepare supper for him. But at the same time her parish welcome wagon is meeting. Which is God’s will for her at that particular moment in time? Spiritual directors and parish priests discern God’s will for us through the hierarchy of truths, because God’s will is precisely that we penetrate deeply into His teachings, reflect on them, and then do our humble best to obey. At a higher level, Rabbi Yeshua has called us to be Jn 10:16 one flock with one shepherd. His vicars, each pope in his time, teach infallibly on faith and morals.
Rabbi Yeshua will lead his vicars to fulfill his holy will for us. The Church’s love for Rabbi Yeshua Jn 10:11, and her obedience to her Shepherd Jn 15:14 through our papa, is what makes us Catholic.
Consider Les Miserables 1:58:08. After Jean Valjean stole the bishop’s precious silver, the bishop told the police that he had given it to Valjean. Did the bishop sin? Not at all. A lie damages the person lied to. The bishop’s purpose was to save Valjean’s soul.
And was it really a lie? The police were ready to give the bishop’s silver back to him but the bishop refused, so he told the police the truth, he did freely give Valjean the silver. “By the witness of the martyrs, by the passion and the blood, God has raised you out of darkness: I have bought your soul for God.”
There are many circumstances in which telling the absolute truth would be the higher virtue. In criminal trials, for instance, the jury needs to have an accurate understanding of what occurred to give a just verdict. Being charitable to a serial offender by letting him off might allow him to molest someone else, so that putting him in prison is a charity to others.
Father William Most, a world-class Scripture scholar and theologian who was one of my own mentors, has written a note, The Hierarchy of Truths and the Truth, well worth reading for anyone who wishes to penetrate more deeply into the hierarchy of truths. In brief summary, the hierarchy does not mean that some “lower level truths” are expendable. Rather, some truths may take precedence over others in particular circumstances, but all that is de fide remains de fide.
This hierarchy of truths does not mean that the Catholic Church will admit even the smallest error into her teaching. She has always recognized different levels of theological certainty. Protected by her papa’s infallible teaching authority, she constantly strives to bring the kingdom of God into the world.