The mediation of Mary in the economy of salvation

We have but one Mediator, as we know from the words of
the Apostle: “For there is one God, and one Mediator between
God and humanity, himself human, Christ Jesus, who gave
himself as a ransom for all” (1 Tim. 2:5-6). The maternal duty
of Mary toward humanity in no way obscures or diminishes
this unique mediation of Christ, but rather shows its power. For
all the saving influences of the Blessed Virgin on people originate,
not from some inner necessity, but from the divine pleasure.
They flow forth from the superabundance of the merits of
Christ, rest on His mediation, depend entirely on it, and draw
all their power from it. In no way do they impede the immediate
union of the faithful with Christ. Rather, they foster this union.
The Blessed Virgin was predestined to be the Mother of God
in the eternal plan for the incarnation of God’s Word. By decree of
God’s providence she was, here on earth, the loving
mother of the divine Redeemer, the noblest of all his companions,
and the humble servant of the Lord. In conceiving Christ,
in bearing him, in nursing him, in presenting him to the Father
in the temple, in sharing her Son’s passion, as he was dying on
the cross, by her obedience, her faith, her hope and burning love,
she cooperated in a way that was unique, in the work of
the Saviour in restoring supernatural life to souls. She is therefore,
a mother to us in the order of grace.
This motherhood of Mary in the order of grace—from the
consent which she gave in faith at the annunciation, and which
she continued to give unhesitatingly at the foot of the cross—
lasts without interruption until all the elect enter into eternal
fulfilment. When she was taken up into heaven, she did not lay
aside this saving role but she continues by her intercession for
all to gain for us the gifts of eternal salvation.
In her maternal love she cares for the brothers and sisters of
her Son as they journey on earth in the midst of dangers and
hardships, until they are brought safely home to the happiness of heaven.
The Blessed Virgin is thus invoked in the Church under the
titles of Advocate, Auxiliatrix, Adjutrix and Mediatrix. These
titles must not, however, be understood as in any way detracting
from, or adding to, the dignity and effectiveness of Christ, the one Mediator.
No creature can ever be classed as an equal with the incarnate Word,
the Redeemer. But just as the priesthood of Christ
is shared in various ways by his ministers and his faithful people,
and as the goodness of God, one though it is, is, in different ways,
really shared with creatures, so also the unique mediation of Christ
does not exclude but brings about a variety of
shared cooperation, deriving from the one unique source.
The Church does not hesitate to acknowledge this kind of
subordinate role in the person of Mary. The Church has
continuous experience of its effects, and commends it to the hearts
of the faithful, so that they, as they lean on her motherly protection,
may be brought into closer union with the Mediator,
our Saviour.

From ‘Lumen Gentium’, the Dogmatic Constitution on the
Church of Vatican II (nn. 60-62. Abbott)