Psalm 51:5 Douay-Rheims Bible
For behold I was conceived in iniquities; and in sins did my mother conceive me.
That is from a Psalm, written by David, about a thousand years before Mary gave birth to Jesus Christ, and it was written by David about himself, and his great contrition for his sins and the evil he had done. (Jesus Christ did not sin, and Jesus Christ did no evil. Jesus Christ was not "conceived in iniquities"; and the Mother of Jesus Christ did not "conceive Jesus Christ in sins".)
This is a case where it actually might be helpful to "post it ALL" (i.e., the whole chapter of Psalm 51), as it is all actually relevent to the matter being discussed here. Here is that Psalm:
Have mercy on me, O God, according to thy steadfast love; according to thy abundant mercy blot out my transgressions.
Wash me thoroughly from my iniquity, and cleanse me from my sin!
For I know my transgressions, and my sin is ever before me.
Against thee, thee only, have I sinned, and done that which is evil in thy sight, so that thou art justified in thy sentence and blameless in thy judgment.
Behold, I was brought forth in iniquity, and in sin did my mother conceive me.
Behold, thou desirest truth in the inward being; therefore teach me wisdom in my secret heart.
Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.
Fill me with joy and gladness; let the bones which thou hast broken rejoice.
Hide thy face from my sins, and blot out all my iniquities.
Create in me a clean heart, O God, and put a new and right spirit within me.
Cast me not away from thy presence, and take not thy holy Spirit from me.
Restore to me the joy of thy salvation, and uphold me with a willing spirit. Then I will teach transgressors thy ways, and sinners will return to thee.
Deliver me from bloodguiltiness, O God, thou God of my salvation, and my tongue will sing aloud of thy deliverance.
O Lord, open thou my lips, and my mouth shall show forth thy praise.
For thou hast no delight in sacrifice; were I to give a burnt offering, thou wouldst not be pleased.
The sacrifice acceptable to God[e] is a broken spirit; a broken and contrite heart, O God, thou wilt not despise.
Do good to Zion in thy good pleasure; rebuild the walls of Jerusalem, then wilt thou delight in right sacrifices, in burnt offerings and whole burnt offerings; then bulls will be offered on thy altar.
Psalm 51
I would like to point out that when Jesus was baptized...the Holy Spirit alighted on him like a dove with the voice of God thundering...”This is my Beloved son in whom I am well pleased!” From then on the baptisms were to be of Jesus Christ which would symbolize the burial of the old man and the rising of the new man by the energizing work of the Holy Spirit! From then on the influence of John the Baptist would decrease while the influence of Jesus Christ would increase as John himself had declared it. Christ would indeed begin to baptize men with “fire and with the Holy Spirit”!
As for Mary, she was in obedience to the Law but she did not fulfill the law...the law was only fulfilled thru the person of Jesus Christ.