A load of old nonsense is talked about the idea of "'heirarchy' of truths" by many who do not really understand Catholicism
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.It is not that some "truths" are less true than others, or that some "truths" can be dispensed with but rather that truth itself is heirarchic. The doctrine of the Trinity, for example, is dependant on the doctrine that God exists, the doctrine the doctrine of the Incarnation is dependant on the doctrine of the Trinity, part of which says God is active in his creation.
Unitatis Reditegratio 34
In the same way the doctrine that the Catholic Church is the "True Church" is dependant on a whole number of other doctines, for example: the acceptance that Jesus Christ is God, that he speaks the Truth, that his words are unchanging, that scripture is true, that the Holy Spirit does indeed guide the Church, all these come together stating quite clearly that the Church was built by God himself on the Rock at Caesarea Philipi and has been guided by the Holy Spirit from the first Pentecost.
It is not that any doctrine is more true or less true than any other but rather that Catholicism is an integral whole, for so many doctrines are interrelated. Take one away, or damage it, and the whole edifice begins to crack and collapses. An obvious example that seems prevalent at the moment, damage the doctrine of the Real Presence and the whole Catholic concept of the intimacy of God with his people is seriously wounded. The Real Presence gives us an understanding of the doctrine of God that is significantly different from a Protestant understanding of the doctrine of God.