Protestants need not worry about Catholics worshipping Christ's mother and the saints. It's taken us some 2,000 years to figure out four dogmatic truths about the Blessed Virgin Mary—that she is the Mother of God, a perpetual Virgin, immaculately conceived, and assumed into heaven. With her husband St. Joseph, we haven't even reached first base. I sometimes think that many Catholics have shifted any discomfort they might have with the ramifications of God choosing to be born into a human family away from Mary and onto Joseph.
Here's one example of the phenomenon making the rounds of Facebook right now:
"Everything that went wrong had to have been Joseph's fault! Poor Joseph," goes a common Catholic cry, evidently not realizing that "mistakes" is not a synonym for "sins." The Virgin Mary could have gotten back late from her many errands of mercy in the village of Nazareth and not had a piping hot dinner on the table for her men when they came home from a hard day's work. Jesus could have incorrectly measured a wooden beam for a project Joseph was working on and thrown off a whole day's work. Neither Mary nor Jesus would have sinned in making those kind of mistakes, but they still might have needed to apologize to Joseph, husband and father.
"Hold on a minute!" you might be saying. "What is this about Joseph being Jesus' father? Didn't you forget to qualify that with the word foster? Or perhaps you really meant to call Joseph Jesus' guardian?"
It must be noted that calling Joseph Jesus' "foster father" or "guardian" has an honorable Catholic pedigree. Bl. John Paul II, as but one example, called Joseph "Guardian of the Redeemer" (the Latin is Redemptoris Custos, which could also be translated as "Custodian of the Redeemer") in his apostolic exhortation "on the person and mission of Saint Joseph in the life of Christ and the Church." The terms are used in an effort to protect the dogmatic truth that Jesus Christ ultimately has no earthly origin—first and foremost, he is the divine Son of God the Father; he is a divine person, not a human person (a man, yes, but not a human person).
Nonetheless, denying Joseph human fatherhood does a disservice to him and makes vulnerable an important piece of our understanding of the person and mission of Christ himself in the Church.
For if St. Joseph is merely a stepfather, a foster father, a guardian, or a caretaker, then he did not adopt Jesus and therefore is not Jesus' legal father. If Joseph is not Jesus' legal father, then he could not have handed on to Jesus his own Davidic heritage. Whether or not Mary was a member of the House of David (she probably was), it was Joseph's adoption of Jesus that gave Jesus legitimacy, that gave him Joseph's family, that made Jesus the Son of David. By the laws of the time, Jesus' Davidic heritage could be passed to him only by a son of David. If Joseph was not truly Jesus' father, as Jesus' own Mother said that Joseph was (cf. Luke 2:48), then Jesus was not truly the Son of David. The great genealogies in the Gospels of Matthew and Luke that link Jesus to King David (Matt. 1:1-17, Luke 3:23-38) would then be no more than legal fiction.
But that's not all. If there is just one point at which Christianity is distinct from all other revealed religions, it is in the Christian understanding of divine filiation, by which we understand that Christ became man so that we might share in the divine life of the Trinity:
The Word became flesh to make us "partakers of the divine nature": "For this is why the Word became man, and the Son of God became the Son of man: so that man, by entering into communion with the Word and thus receiving divine sonship, might become a son of God." "For the Son of God became man so that we might become God." "The only-begotten Son of God, wanting to make us sharers in his divinity, assumed our nature, so that he, made man, might make men gods" (CCC 460).
Or, as John put it more simply in his first epistle, "See what love the Father has given us, that we should be called children of God; and so we are" (1 John 3:1).
Let's suppose for a minute that Joseph's human fatherhood of Jesus is nothing more than a sham God used to make the Holy Family look respectable to their neighbors in Nazareth. Let's suppose that Joseph was no more than the man in the corner of the stable, given tidbits of responsibility here and there but never really a husband or father. If that's the case, then perhaps we should start to wonder what adoption means in the Christian economy. Is it a real adoption that makes us God's children, or is it another legal fiction?
In The Great Heresies, Hilaire Belloc once defined heresy as "the dislocation of some complete and self-supporting scheme by the introduction of a novel denial of some essential part therein." We see most readily that the affirmation of the Blessed Virgin's Motherhood of God is of utmost importance to Christian orthodoxy. I believe we need to begin to make a similar case for the fatherhood of St. Joseph.
Joseph is visited by the messenger as "Mary's spouse," as the one who in due time must give this name to the Son to be born of the Virgin of Nazareth who is married to him. It is to Joseph, then, that the messenger turns, entrusting to him the responsibilities of an earthly father with regard to Mary's Son (Bl. John Paul II, Guardian of the Redeemer 3).