" - and a sword will pierce your own soul too."
Mother and son are perfectly united in Christ's passion. Mary was there for all of it, right up to the foot of the Cross (John 19. 25), right up to the piercing of her son's dead body (John 19.34). Christ's body was pierced, Mary's soul was pierced. How are we to imagine what it is like to have our soul pierced? Even if we could imagine it Mary suffered far more, for her soul is the soul not just of Christ's human body. To Mary belongs the soul of the Mother of God. She was capable, therefore, of suffering exquisitely more than we can possibly fathom.
Jesus binds his mother so closely to himself that when a sword pierces him it passes through her as well. Mother and son are united like joint and marrow. The son's death splits them asunder. Yet Christ lays down his own life, nobody takes it from him. And so we read that the word of God pierces until it divides the joint from the marrow (Hebrews 4.12).
Blood and water spill out from the son's pierced side. When the mother's soul is pierced with the sword which separates her from her son, our own consciences spill out - our own thoughts. This was foretold by Simeon, "so that the inner thoughts of many will be revealed." (Luke 2.35) The Word of God which pierces until it divides soul from spirit, joint from marrow, (mother from son) then judges the thoughts and intentions of the heart (Hebrews 4.12).
With cries that pierce me to the heart,
my enemies revile me,
saying to me all the day long:
"Where is your God?" (Psalm ?)