Some Christians believe that the Immaculate Conception is incompatible with the unique holiness of God and the redemptive power of Christ. Given the gravity of the original sin and its universal impact, no one should belittle the importance of Christ's sacrifice on the Cross by saying that it was not enough.
But this is not the case with the Immaculate Conception. Non-Catholics are under the impression that the Immaculate Conception indicates that Mary deserved this honor, and that her holiness came from her own actions or from the mere fact of her existence. This is completely false and does not represent the teaching of the Catholic Church.
While all men are born with the stain of original sin (from which we are saved by faith and baptism into Christ), the Church teaches that Mary was sanctified by God's grace at the moment of her conception in her mother's womb, and that this "absolutely unique radiant holiness" with which she was "enriched from the first moment of her conception" (Lumen Gentium § 56) came to her entirely from Christ: she was "eminently redeemed in consideration of the merits of her Son" (LG 53). More than any other created person, the Father "blessed her with every spiritual blessing in heaven in Christ" (Eph 1:3). He "chose her in him from before the foundation of the world to be holy and immaculate in his presence, in love" (Catechism of the Catholic Church § 492).
Adapted from an article by René Albert for Patheos, July 2020