Mary, as the New Eve, gave the world the instrument of redemption–the body of Christ, just as Eve gave the world the instrument of the Fall–the fruit of the Tree of Knowledge. In this sense, Mary is called co-redemptrix, due to her participation in the Redemption. But Mary is also called co-redemptrix in the sense that she offered her Son Jesus, with her maternal rights, in union with her own sufferings at witnessing the Passion of Jesus, for the salvation of humanity. By this act, Mary herself merited the grace of redemption for all mankind. This was foreshadowed by Simeon, who said to Mary: "a sword will pierce your own heart, too" (Luke 2:35).
Two things must be stated. First of all, "co" does not mean "equal," but "with." Mary did not play an equal role in the Redemption. Secondly, Mary's role in the Redemption was in every way secondary to and dependant on the work of Christ. Without Christ, there would have been no redemption, and Mary would not have been co-redemptrix.
Protestantss argue by saying that Christ alone is our redeemer. The Bible says: "I, the Lord, am your Savior, and your Redeemer, the Mighty One of Jacob" (Is. 49:26) and "the redemption . . . is in Christ Jesus' (Rom. 3:24). Although none of these verses say Jesus alone redeems, Catholics would agree that it is through Christ alone that the redemption occurred. It is the suffering and death of Jesus Christ alone that brings the redemption of mankind. So how can Mary be co-redeemer if Jesus alone redeems? Mary's sufferings did not redeem mankind, but her sufferings, in union with the suffering and death of Christ, made her a participant in the redemption. The key word is "united". Jesus alone redeems in the sense that only His suffering and death had the power to free man from sin. Mary's sufferings are powerless to redeem, but when united to Christ, her sufferings had redemptive value. If Christ won saving grace for mankind, Mary also won it, because her work was perfectly united to the saving work of Christ.
Think about it this way. There was a man who had erected a fence in his yard. One night, there was a storm. In order to prevent the winds from toppling the fence, the man stood outside and held it. Soon afterwards, his daughters came out to help him. In reality, the strength of the man was enough to keep the pole standing, but the daughters did help to keep the fence standing, but only because of the work of the father. Without him, the fence would have toppled. Yet the girls were praised by the father and rewarded by the father, because they participated in keeping the fence standing. This is precisely what happened at the Redemption. The passion and death of Jesus was sufficient to redeem mankind, but Mary participated and therefore also helped to redeem mankind, though her effort was useless without the work of Jesus.
Protestants also quote Hebrews 9:14, which says that Christ "offered Himself without blemish to God." So it was not Mary that offered Christ, they say, it was Christ Himself. Nowhere does this passage say or imply that Christ offered himself alone without Mary's participation. Christ did offer Himself to God, but there is no contradiction in saying Mary offered Him up as well. Christ offered Himself, and Mary offered Him as well. At the Sacrifice of the Mass, Catholics are still taught to offer Jesus Christ to the Father in union with their own prayers, works, and sufferings. Jesus offers Himself in the Mass, of course, but we participate in that offering.
Protestants argue that Mary experienced a suffering of compassion and a suffering for the sake of righteousness. They say it was different for Christ, who suffered for sin. But this is a blunder. The kind of suffering compared with what was suffered for is like comparing apples and oranges. Yes, Jesus suffered differently than Mary, but their sufferings do not have to be identical in order to both have redemptive value. They both suffered for sin. Protestants say Mary was not aware that her sufferings had redemptive value, but that is simply speculation. They also argue that many others, like Mary Magdalene and the Apostle John, suffered that day. This is true, but it was not God's will that they be co-redeemers. It was God's will that Mary help to redeem mankind. So He accepted her sufferings, the sufferings of a woman "full of grace" in union with the sufferings of Christ in order that she may play a role in the redemption.
Protestants also make the objection that the Redemption is linked to the death of Christ rather than His suffering: "we were reconciled to God through the death of His Son" (Rom. 5:10); "a death has taken place for the redemption of the transgressions" (Heb. 9:15); "He . . . released us from our sins by His blood" (Rev. 1:5). This is true, but His sufferings were in effect a part of His death. Both His suffering and death were offered as one sacrifice. Both were offered in satisfaction for the sins of man.
Protestants object to this by saying that since death was a penalty for sin (Gen. 2:17, Rom. 6:23), a life had to be given to redeem us. Of course, Protestants forget that suffering is also a penalty for sin (Gen. 3). Mary's sufferings, when united with Christ's act of redemption (His suffering and death) make her a participant in the redemption. She didn't have to die. Her sufferings were united to the redemptive work of Christ.
Protestants sometimes use the straw man that Christ alone was qualified to redeem mankind. Of course, Christ alone was qualified. Mary was powerless to redeem mankind. The Catholic Church just teaches that she participated in the redemption, not that she redeemed man herself or could have done so without Christ. They also say Christ alone deserves the title of "redeemer." Of course, this is true. The Catholic Church has never given Mary the title of "redeemer," only "co-redeemer," which is much different. "Co" means "alongside" or "with," and does not mean "equal" or even imply "equal". A "co-worker" is one that works alongside another. This could be someone of equal authority, but it could someone in authority or under the authority of another.