Mary's cooperation in the redemption
The title "Coredemptrix" which has been current since the fifteenth century and which has appeared in some official church documents under Pius X, cannot be concieved of as an equasion of Mary's activity with the redemptive activity of Chirst. She required redemption in the same manner as every person. Her coopration with the redemption is an indirect, supportive, remote cooperation and, it derives from this, that she voluntarily devoted her whole life to the Redeemer and, under the Cross, suffered and sacrificed with him.
Pius XII said in "Mystici Corporis", "...she offered Him on Golgatha to the Eternal Father together with the holocost of her maternal rights and her motherly love like a new Eve for all her children, the children of Adam."
Chirst alone, according to Catholic doctrine, is the Redeemer of humanity. Mary merely offered Him moral support for His action. Her words in Luke 1,38 : "Behold the Handmaid of the Lord," imply support for and remote co-operation in the redemption. St. Ambrose expressly teaches, " Christ passion did not require any support." (De inst. virg. 7).
In the power and grace of the redemption, Mary, by spiritually entering into the sacrifice of her Divine Son, made attonement for the sins of men and (de congruo) merited the application of the redemptive grace of Christ. In this manner she cooperates in the subjective redemption of mankind. In this sense only can she be concieved of a "Coredemptrix".