3 The special motives for which St. Joseph has been proclaimed Patron of the Church, and from which the Church looks for singular benefit from his patronage and protection, are that Joseph was the spouse of Mary and that he was reputed the Father of Jesus Christ. From these sources have sprung his dignity, his holiness, his glory. In truth, the dignity of the Mother of God is so lofty that naught created can rank above it. But as Joseph has been united to the Blessed Virgin by the ties of marriage, it may not be doubted that he approached nearer than any to the eminent dignity by which the Mother of God surpasses so nobly all created natures. For marriage is the most intimate of all unions which from its essence imparts a community of gifts between those that by it are joined together. Thus in giving Joseph the Blessed Virgin as spouse, God appointed him to be not only her life's companion, the witness of her maidenhood, the protector of her honour, but also, by virtue of the conjugal tie, a participator in her sublime dignity. And Joseph shines among all mankind by the most august dignity, since by divine will, he was the guardian of the Son of God and reputed as His father among men. Hence it came about that the Word of God was humbly subject to Joseph, that He obeyed him, and that He rendered to him all those offices that children are bound to render to their parents. From this two-fold dignity flowed the obligation which nature lays upon the head of families, so that Joseph became the guardian, the administrator, and the legal defender of the divine house whose chief he was. And during the whole course of his life he fulfilled those charges and those duties. He set himself to protect with a mighty love and a daily solicitude his spouse and the Divine Infant; regularly by his work he earned what was necessary for the one and the other for nourishment and clothing; he guarded from death the Child threatened by a monarch's jealousy, and found for Him a refuge; in the miseries of the journey and in the bitternesses of exile he was ever the companion, the assistance, and the upholder of the Virgin and of Jesus. Now the divine house which Joseph ruled with the authority of a father, contained within its limits the scarce-born Church. From the same fact that the most holy Virgin is the mother of Jesus Christ is she the mother of all Christians whom she bore on Mount Calvary amid the supreme throes of the Redemption; Jesus Christ is, in a manner, the first-born of Christians, who by the adoption and Redemption are his brothers. And for such reasons the Blessed Patriarch looks upon the multitude of Christians who make up the Church as confided specially to his trust - this limitless family spread over the earth, over which, because he is the spouse of Mary and the Father of Jesus Christ he holds, as it were, a paternal authority. It is, then, natural and worthy that as the Blessed Joseph ministered to all the needs of the family at Nazareth and girt it about with his protection, he should now cover with the cloak of his heavenly patronage and defend the Church of Jesus Christ.
Source: Quamquam Pluries (Vatican.va)