Why is it that devotion to Our Lady unites us with Christ? It is quite simply because She is the way established by God for us to reach Him.
“The soul that would reach transforming union in the Divinity of Christ must wholly apply itself to copying the Sacred Humanity of Christ”, says St. John of the Cross. But God willed from all eternity that Our Lord Jesus Christ be sent to mankind for its redemption through a woman. He willed to depend upon her for the work of Redemption. Strictly speaking, of course He needed no one to accomplish His plans. However, the facts are that He chose to depend on Mary for:
During: the bringing of the God-man to earth and the accomplishment of His redeeming sacrifice (Our Lady’s role as Co-Redemptrix)
After: the sanctification of souls by the application of the fruits of Redemption to them and the leading of them to Heaven
And God never changes. He willed that our redemption be accomplished through Mary at the coming of Our Lord. He wills today likewise that our redemption be accomplished through her. He willed that the Head of the Mystical Body be formed within her. He wills that the members be formed in her as well.
By the will of God, the path by which we are get to Jesus Christ and to Him is Our Lady.
What can we do but admire this plan and want to conform ourselves to it? What a beautiful plan it is! How well adapted to our poor human nature! We must contemplate it and love it. God wanted to make a mother as an indispensable condition for salvation. He willed to be dependent in order that we would have a mother for our salvation, in order that we would not be as children with no mother, in order that he would not be providing us more physically than spiritually, giving us physical father and mother, but no spiritual mother.
Thus, we can conclude that devotion to Our Lady is necessary for us to reach Our Lord. This is what Archbishop Lefebvre remarks:
“Devotion to the Blessed Virgin is not a devotion like that we might have for one or another saint. Devotion to the most Blessed Virgin Mary is a necessary devotion because God willed it to be so. He could have come among us without passing by the Blessed Virgin, obviously. He could have found another means, but He chose that one. We do not have the right to argue with it and on the contrary we should thank Him, and thank Him infinitely, for having given us a mother. The Blessed Virgin Mary is a mother who takes care of us and who follows the Church in her difficulties... How could we not have an immense devotion to the most Blessed Virgin Mary?” – Priestly Holiness, p. 290.
How important it is for us, then, to embrace the True Devotion to Mary as proposed by St. Louis de Montfort.