The Immaculate Conception of the Holy Theotokos

The Holy Mother of God, by virtue of the saving grace of Christ's Sacred Death and Glorious Resurrection, received from the moment of HER conception (in the womb of her mother, St. Anne) the justification that we receive in Holy Baptism. While not being the actual words of the dogma, this is what it teaches. The grace of Holy Baptism is incapable of being purchased or earned. It is given to us freely by God in response to faith.
The readings for today present Adam and Eve and the story of the fall. Man lived in a state of innocence, i.e., "not knowing" either good or evil. With the fall, they knew that they were naked. They had learned evil. God's plan was frustrated by the freedom of will that he had endowed us with. There had to be a remedy.
The death and resurrection of Christ was able to work salvation in folks after those historical events took place. But they also worked backwards to grant justification to those who went before. Think of the holy worthies of the Old Testament: Abraham, Isaac, Jacob, Sarah, Rebeca, Rachael, Moses, Joshua, Aaron, Miriam, etc. Then consider those others, too many to be named and unknown to us who followed God as best they could. At the moment of Christ's death, they received the justification that was needed for them to be completely joined to God. So we can see from that they the grace of the "Paschal Mystery" can work backward in time, for it is only by virtue of this event that any can be saved.
But if Christ's saving grace can work backward, then it could also work forward, in the sense that it could be available to for God's divine purpose at any time. Thus, in order that the freely-chosen sin of Eve could be overturned by an act of freely-chosen obedience, the Immaculate Conception fits the bill.
In order for the Theotokos to be free to choose to say "yes" to God, she had to be free from the effects of sin. When we choose, it is always from our selfishness. Even when we choose for God, we still want to know what's in it for us. It is only by cooperation with grace that we can even choose to do the good and avoid evil at all. By giving St. Mary the Virgin at her conception the justification that he won for our salvation, Christ, frees her without the sacrament of Baptism (that didn't exist then) through the grace of his sacrifice that is available at all times to fulfill the Father's purpose.
Collect: Father, you prepare the Virgin Mary to be the worthy mother of your Son. You let her share beforehand in the salvation Christ would bring by his death, and kept her sinless from the first moment of her conception. Help us by her prayers to live in your presence without sin. We ask this through our Lord Jesus Christ, your Son, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen