5 But the first and chief reason, Venerable Brethren, why the fiftieth anniversary of the proclamation of the dogma of the Immaculate Conception should excite a singular fervour in the souls of Christians lies for us in that restoration of all things in Christ which we have already set forth in Our first Encyclical letter. For can anyone fail to see that there is no surer or more direct road than by Mary for uniting all mankind in Christ and obtaining through Him the perfect adoption of sons, that we may be holy and immaculate in the sight of God? For if to Mary it was truly said: "Blessed art thou who hast believed because in thee shall be fulfilled the things that have been told thee by the Lord" ( Luke i., 45); or in other words, that she would conceive and bring forth the Son of God and if she did receive in her breast Him who is by nature Truth itself in order that "He, generated in a new order and with a new nativity, though invisible in Himself, might become visible in our flesh" (St. Leo the Great, Ser. 2, De Nativ. Dom .): the Son of God made man, being the "author and consummator of our faith"; it surely follows that His Mother most holy should be recognized as participating in the divine mysteries and as being in a manner the guardian of them, and that upon her as upon a foundation, the noblest after Christ, rises the edifice of the faith of all centuries.
Source: Ad Diem Illum Laetissimum (Vatican.va)