The Heart of the Gospel

36. All revealed truths derive from the same divine source and are to be believed with the same faith, yet some of them are more important for giving direct expression to the heart of the Gospel. In this basic core, what shines forth is the beauty of the saving love of God made manifest in Jesus Christ who died and rose from the dead. In this sense, the Second Vatican Council explained, "in Catholic doctrine there exists an order or a 'hierarchy' of truths, since they vary in their relation to the foundation of the Christian faith." This holds true as much for the dogmas of faith as for the whole corpus of the Church's teaching, including her moral teaching. 

The Joy of the Gospel, Pope Francis, par. 36, p. 19

The Fact of Christ's Birth Organizes the Universe Around a New Axis

...Christianity is not something  verbal, something that belongs to the sphere of words. Neither is it a doctrine or,...a message. It presents itself...as a fact...as something that happened, as much more than a verbal message that was written down and that would count as a doctrine, or even a system. Msgr. Giussani does not speak of Christianity but of what he calls the Christian fact. ... what we are dealing with in Christianity is not a doctrine, but a person who must be taken as such and for whom one must make space in one's life.  To convey this idea Giussani uses a splendid metaphor describing how the birth of a child forces the entire universe of a family to reorganize: "When a child is born into a family, it is clear to the parents, to the grandparents, to the whole family and to friends that this is a fact. There is nothing to dispute: a new bed is needed. Perhaps another bedroom; we have to give thought to how to take care of the new arrival; we are concerned about feeding, clothing, and protecting him or her; we get up at night if he or she needs us. The shape of daily life is transformed by virtue of this fact...Christianity is a fact in the very same way. It entered history just as a child enters the house of a husband and wife. Christianity is an irreducible event, an objective presence that desires to reach man; until the very end it means to be a provocation to him and to offer a judgment of him." 

 A Generative Thought, edited by Elisa Buzzi, Chapter Two: Christianity: a Fact in History,  p.35