Mediatrix and co-redemptrix are terms that will certainly bother our Protestant brothers and sisters. The terminology is Catholic and has to be explained by Catholics because the words mean something different to non-Catholics.
In this regard it is important to look at the meaning of “the one mediator between God and Man.” Every Catholic would agree with our Protestant brothers and sisters that this statement refers to Jesus alone. We all know that only the blood of Jesus was shed for our redemption. Only Jesus died on the cross. Only Jesus could provide the propitiating sacrifice to God because only God in the flesh could provide a sacrifice of infinite worth. That having been said, non-Catholics should be able to see that Catholics mean something different when they refer to Mary with the aforementioned terms.
In scripture we read about being co-workers with Christ. This is evidenced by the following verses:
Rom 16:9
Greet Urbanus, our co-worker in Christ, and my beloved Stachys.
1 Cor 3:9
For we are labourers together with God: ye are God’s husbandry, [ye are] God’s building.
2 Cor 6:1
As we work together with him, we urge you also not to accept the grace of God in vain.
3 Jn 1:8
Therefore we ought to support such people, so that we may become co-workers with the truth.
As co-workers with Jesus we are in no way equal to Jesus. So it is with Mary. She is given the aformentioned titles in recognition of her unique and predestined role in God’s plan of salvation. No other co-worker in Christ shares her unique position or role and so her titles are appropriate, and they are in keeping with scriptures descriptions of all Christians.
In Colossians 1:24 the apostle, Paul, makes the statement, “Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ’s afflictions for the sake of his body, that is, the church.” This verse should be very upsetting to anyone that gets nervous about the titles the Church places on Mary. So far I have not heard anyone condemn Paul for having said this.