The Immaculate Conception and the Mediation of Christ

Mary did not contract original sin, because of the excellence of her Son, inasmuch as He is Redeemer, Reconciler, and Mediator. For the most perfect mediator would perform the most perfect act of mediation on behalf of any person for whom he mediated. And Christ is the most perfect Mediator. Therefore, Christ showed the most perfect possible degree of mediating with respect to any creature or person whose mediator He was. But for no other person did He exhibit a more excellent degree of mediation that He did for Mary...But this would not have happened if He had not merited that she should be preserved from original sin.