The divine household, which Joseph governed, just as with paternal authority, contained a beginnings of the new Church. The Virgin most holy is the mother of all Christians since she is the Mother of Jesus and since she gave birth to them on the mount of Calvary amid the unspeakable sufferings of the Redeemer. Jesus is, as it were, the firstborn of all Christians, who are His brothers by adoption and redemption.
From these considerations we conclude that the blessed patriarch (St. Joseph) must regard all the multitude of Christians who constitute the Church as confided to his care in a certain special manner.
This is his numberless family, scattered throughout all lands, over which he rules with a sort of paternal authority, because he is the husband of Mary and the father of Jesus Christ. Thus it is conformable to reason and in every way becoming to Blessed Joseph that as once it was his sacred trust to guard with watchful care the family of Nazareth, no matter what befell, so now, by virtue of his heavenly patronage he is in turn to protect and to defend the Church of Christ.
by Pope Leo XIII On the Devotion to St. Joseph