De Montfort:
20. God the Holy Spirit, who does not produce any divine person, became fruitful through Mary whom he espoused. It was with her, in her and of her that he produced his masterpiece, God-made-man, and that he produces every day until the end of the world the members of the body of this adorable Head. For this reason the more he finds Mary his dear and inseparable spouse in a soul the more powerful and effective he becomes in producing Jesus Christ in that soul and that soul in Jesus Christ.
21. This does not mean that the Blessed Virgin confers on the Holy Spirit a fruitfulness which he does not already possess. Being God, he has the ability to produce just like the Father and the Son, although he does not use this power and so does not produce another divine person. But it does mean that the Holy Spirit chose to make use of our Blessed Lady, although he had no absolute need of her, in order to become actively fruitful in producing Jesus Christ and his members in her and by her. This is a mystery of grace unknown even to many of the most learned and spiritual of Christians.