Mary’s faith unties the knot of sin (cf. Lumen Gentium, 56). What does that mean? The Fathers of the Second Vatican Council took up a phrase of Saint Irenaeus, who states that “the knot of Eve’s disobedience was untied by the obedience of Mary; what the virgin Eve bound by her unbelief, the Virgin Mary loosened by her faith” (Adversus Haereses, III, 22, 4).
The “knot” of disobedience, the “knot” of unbelief. When children disobey their parents, we can say that a little “knot” is created. This happens if the child acts with an awareness of what he or she is doing, especially if there is a lie involved. At that moment, they break trust with their parents. How often does this happen! Then the relationship with their parents needs to be purified of this fault; the child has to ask forgiveness so that harmony and trust can be restored. Something of the same sort happens in our relationship with God. When we do not listen to him, when we do not follow his will, we do concrete things that demonstrate our lack of trust in him – for that is what sin is – and a kind of knot is created deep within us. These knots take away our peace and serenity. They are dangerous, since many knots can form a tangle which gets more and more painful and difficult to undo.
But nothing is impossible for God’s mercy! Even the most tangled knots are loosened by his grace. And Mary, whose “yes” opened the door for God to undo the knot of the ancient disobedience, is the Mother who patiently and lovingly brings us to God, so that he can untangle the knots of our soul by his fatherly mercy. We might ask ourselves: What knots do I have in my life? Do I ask Mary to help me trust in God’s mercy, in order to change?
— Pope Francis’ catechesis on “The Faith of Mary,” October 12, 2013 (full text here)