BTW, this is an incorrect view of the atonement, one popular among Protestants but which Catholics rightfully reject.
It actually minimizes what Christ really did, and makes the atonement less that what it was.
St. Anselm argued that every sin is an infinite insult to the Divine majesty, and requires an infinite propitiation. He further argues that only an infinite person, that is, a Divine person, could offer an infinite propitiation.
Hence the atonement did not merely require a person who was sinless, but a Person who is Divine.
This is aside from the fact that Mary was sinless only through God's grace, not by her own power. According to St. Anselm's argument, even if there had been a human person available who was without sin by their own power (notice: Mary does not qualify), his death would not have been sufficient to completely atone for the sins of anyone else, much less everyone else.
578 Jesus, Israel's Messiah and therefore the greatest in the kingdom of heaven, was to fulfill the Law by keeping it in its all embracing detail - according to his own words, down to "the least of these commandments".330 He is in fact the only one who could keep it perfectly.331 On their own admission the Jews were never able to observe the Law in its entirety without violating the least of its precepts.332 This is why every year on the Day of Atonement the children of Israel ask God's forgiveness for their transgressions of the Law. The Law indeed makes up one inseparable whole, and St. James recalls, "Whoever keeps the whole law but fails in one point has become guilty of all of it."333
579 This principle of integral observance of the Law not only in letter but in spirit was dear to the Pharisees. By giving Israel this principle they had led many Jews of Jesus' time to an extreme religious zeal.334 This zeal, were it not to lapse into "hypocritical" casuistry,335 could only prepare the People for the unprecedented intervention of God through the perfect fulfillment of the Law by the only Righteous One in place of all sinners.336
580 The perfect fulfillment of the Law could be the work of none but the divine legislator, born subject to the Law in the person of the Son.337 In Jesus, the Law no longer appears engraved on tables of stone but "upon the heart" of the Servant who becomes "a covenant to the people", because he will "faithfully bring forth justice".338 Jesus fulfills the Law to the point of taking upon himself "the curse of the Law" incurred by those who do not "abide by the things written in the book of the Law, and do them", for his death took place to redeem them "from the transgressions under the first covenant".339
Why was Jesus the ONLY one that could keep the whole law? Why was it important to have the only one that could keep the whole law be THE ONE that died on the cross?
Define Atone and define redeem
Show me in scripture where The apostles taught the atonement must be by a "divine person,"( other than the fact that the law reflects the nature and character of God and ONLY HE COULD KEEP IT PERFECTLY)
Jesus never said he only died to "Open the gate of heaven so that men might be able to be saved He said HE was the Gate and NO ONE could enter but through him , he said nothing of their works or righteousness finishing what he started