6 About the words of Simeon to Mary, there is no obscurity or variety of interpretation. “And Simeon blessed them, and said unto Mary His mother, Behold, this Child is set for the fall and rising again of many in Israel; and for a sign which shall be spoken against; (yea, a sword shall pierce through your own soul also,) that the thoughts of many hearts may be revealed.” Here I am astonished that, after passing by the previous words as requiring no explanation, you should enquire about the expression, “Yea, a sword shall pierce through your own soul also.” To me the question, how the same child can be for the fall and rising again, and what is the sign that shall be spoken against, does not seem less perplexing than the question how a sword shall pierce through Mary's heart.
Source: Letters (New Advent)