Wondering what's the general opinion here about the idea of Mary as our Co-Redemptrix. There is a huge push for this to be defined as the fifth Marian dogma, so what do you think? Is it just a modern novelty born out of excess or is it actually a notion with some sort of foundation in the deposit of faith?
Not a novelty: not at all. This ultimately has its foundation upon the teaching of the Fathers regarding the Blessed Virgin Mary as the Second Eve: please consult the following thread:
http://www.cathinfo.com/catholic.php/The-Blessed-Virgin-Mary-the-Second-EveIt must be remembered that the role of the Blessed Virgin Mary in the Mysteries of the Redemption and in the economy of salvation is absolutely singular and unique: the Divine Maternity to which she had been predestined from all eternity, and all its the concomitant and consequent glories and graces, made her a world unto herself.
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The theologians of the 19th century warmly disputed the question pertinent to this discussion, so that in the theological manuals of Fr. Pohle and Fr. Scheeben, one may detect a certain suspicion on the doctrine of Our Lady as Co-Redemptress, found in the writings of Sts. Alphonsus and Louis-Marie, amongst others. This was mostly because these theologians were afraid that a sacerdotal character would be superimposed upon the Blessed Virgin Mary, who never received, nor was capable or receiving, the Sacrament of Holy Orders, as all other women are.
However, since the reign of Pope St. Pius X, who sanctioned the devotion to Our Lady as Co-Redemptress, and with the elucidation of eminent Mariologists following the definition of the dogmas of the Immaculate Conception and corporeal Assumption of the Blessed Virgin Mary and various key Encyclical Letters of the Sovereign Pontiffs and the introduction of new Offices in the Roman Missal and Breviary (e.g., the Immaculate Heart, the Queenship, &c.), the general consensus of the theologians regarding this matter has changed very much.
In his work
The Mother of The Savior and Our Interior Life (Part II, chap. ii., art. 3; trans. Rev. Father Bernard J. Kelley; Dublin: Golden Eagle Books, Ltd., 1948), Rev. Fr. Reginald Garrigou-Lagrange expounds upon this matter most lucidly and elegantly, and represents the general consensus of both theologians and the faithful.