Although the Holy Mother of God is called 'mediatrix' (mesitria) in the writings of the Church Fathers and hymns of the Church, this is never anywhere in the sense of co-mediatrix or co-redemptrix but only in the sense of Intercessor. Mary Immaculate is indeed an Intercessor, the greatest in the Communion of Saints, the New and Second Eve and Mother of all the living, but not a co-redeemer.
'And chiefly in the glorious and most Blessed Virgin Mary, Mother of thy Son Jesus Christ our Lord and God' (1549 English Prayer Book).
O higher than the cherubim,
more glorious than the seraphim,
lead their praises, Alleluia!
Thou Bearer of th' eternal Word,
most gracious, magnify the Lord.(Hymnal 1940, 599)
Such is the patristic consensus. Saint Ambrose of Milan comments on the scene in Saint John 19. Note how Saint Ambrose definitely rules out any notion of Our Lady as a co-redemptrix with Christ’s atoning sacrifice...
‘Perchance because that Royal Hall, the Virgin, knew that the redemption of the world would be through the death of her Son, she thought that by her death also she might add something to the public welfare. But Jesus did not need a helper for the redemption of all, Who saved all without a helper. He received indeed the affection of his Mother, but sought not another’s help.’ (Saint Ambrose of Milan, Letter 63).
