Today is the Feast of the Immaculate Conception, a feast day that always puzzled me even when I was actively trying to convince myself that I was a believing Catholic. Britannica defines it succinctly enough in a non-theological, non-devotional way:

Immaculate Conception, Roman Catholic dogma asserting that Mary, the mother of Jesus, was preserved free from the effects of the sin of Adam (usually referred to as “original sin”) from the first instant of her conception. (Britannica)

The question it at first raises, before the skeptic has a full understanding of the doctrine, is why God could not simply do for all humans what he did for Mary. Why not just preserve all people “from the effects of the sin of Adam” instead of this whole convoluted way of getting forgiveness in the Old Testament through blood sacrifice which then comes to full fruition in the New Testament with an actual human sacrifice (i.e. Jesus)? If he could do it for Mary, why couldn’t he do it for everyone?

A Catholic apologist at this point would explain that it’s not simply that God preserved Mary from the effects of this sin without the need of Jesus and his sacrifice. Instead, the apologist would explain, the sacrifice was applied to Mary in some kind of retroactive way. The Catholic Encyclopedia New Advent explains it thusly (emphasis added)

The immunity from original sin was given to Mary by a singular exemption from a universal law through the same merits of Christ, by which other men are cleansed from sin by baptism. Mary needed the redeeming Saviour to obtain this exemption, and to be delivered from the universal necessity and debt (debitum) of being subject to original sin. The person of Mary, in consequence of her origin from Adam, should have been subject to sin, but, being the new Eve who was to be the mother of the new Adam, she was, by the eternal counsel of God and by the merits of Christ, withdrawn from the general law of original sin. Her redemption was the very masterpiece of Christ’s redeeming wisdom. He is a greater redeemer who pays the debt that it may not be incurred than he who pays after it has fallen on the debtor. (New Advent)

Yet far from making this a simpler solution that solves the question of why God didn’t just do this for everyone, it makes an even more convoluted and illogical argument. Somehow, an event that hadn’t yet taken place affected the conception of the person who would later give birth to the individual to whom this salvific event would take place — see, there’s just no way to explain it without it sounding like some kind of theological Rube Goldberg contraption.