On Calvary, Mary united herself to the sacrifice of her Son and made her own maternal contribution to the work of salvation, which took the form of labour pains, the birth of the new humanity.
In addressing the words “Woman, behold your son” to Mary, the Crucified One proclaims her motherhood not only in relation to the Apostle John but also to every disciple. The Evangelist himself, by saying that Jesus had to die “to gather into one the children of God who are scattered abroad” (Jn 11:52), indicates the Church’s birth as the fruit of the redemptive sacrifice with which Mary is maternally associated.
from the September 17, 1997 General Audience of Pope John Paul II