Someone on that homeschooling forum I hang out on has asked “Just reading through this thread and wondering….Why is important for RCCs to believe that Mary is sinless?  I don’t agree with that I’m not really looking for an argument about whether she is or now, but just wondering why that is important?  It doesn’t change who Jesus is if she had sin, does it?

It wouldn’t be the first time the Church had affirmed the nature of Jesus by what it teaches about His mother. The reason the Catholic church teaches that she is the Mother of God is because of his fully human and fully divine nature. The two are directly tied. The Nestorian heresy was fought with by adding the phrase, “Mother of God” into common Catholic speech and into a prayer known as the “Angelic Salutation”….although that prayer is more commonly known as the “Hail Mary.”

Part of the scriptural basis for teaching that Mary was immaculately conceived is that Jesus is that as the Bread of Life, He is the fulfillment of Manna. As the Eternal High Priest, He is the fulfillment of the Aaronic priesthood and He is the fulfillment of the law. Each of these items was symbolized by something that was in the old Ark of the Covenant. Manna, the rod of Aaron, and the stone tablets of the law were all in the ol Arl of the Covenant. The Ark of the New Covenant (according to Catholic teaching is Mary). The old ark was made of incorruptible (that is it’s symbolism) accacia wood, the new ark was made (as a gift of God not by any merit of hers) of incorrupted flesh. Just as the Catholic Church asks “If Mary isn’t the “Mother of God”, why not? Is it because Jesus isn’t divine?” Remember that the Catholic Church’s memory is long and diverse, very often she has learned that those who would deny the divinity of Christ or His bodily resurrection or some other essential part of Christological doctrine, start by dissing His mother. (Of course, not always.) In parallel, in the eyes of Catholic theology if Mary is not the new Ark of the Covenent, why not? What part of the contents of the old ark was not fulfilled in her Son, Jesus?

So I think that’s why (in part) in the eyes of Catholic theology, to deny the Immaculate Conception is to call into question the nature of Jesus.

As for why God chose to give her that gift? I don’t think that there is any official reason for that. Certainly there is the perspective that many have being that Jesus needed to occupy a sinless vessel but it’s one that I buy. I don’t think Jesus needed to occupy a sinless vessel at all….after all He seems to be able to embrace and love US in our sinful state. On the other hand in my reflection and meditation on this mystery (and believe me, as a convert to Catholicism, I’ve had to do a LOT of reflection and meditation) has led me to the Old Testament. Time and again, we see that to come into the unveiled presence of God was deadly. Look what happened to those who touched the old Ark of the Covenant. To see the face of God was to die! Even in the New Testament, we see that eating and drinking the Body and Blood of our Lord unworthily causes dam*ation (death). Imagine then, carrying the Living God in your body…the Word of God, the Second Person of the Trinity within your body. How could she have borne it without the gift that God gave her?

No I think that the gift of the Immaculate Conception was to equip Mary for the task for which she was chosen.

You might be interested in these resources. Here is the declaration of the Immaculate Conception  and here an article about the dogma that you might find interesting.