Prevenient Grace, which term the priest spoke today during the prayer over the offerings, is the same as Preventing Grace, and in this instance, relates to the singular grace that Mary received in being conceived without original sin.
Prevenient or Preventing Grace is that grace which God gives without the consent or cooperation of man for the sake of man’s well being. It is a grace necessary in order to motivate us to respond to God, which is another kind of grace: Concomitant Grace. Preventing Grace can be a good thought or inclination to do a good act that leads to heaven while Co-operating Grace is the assistance given by God to complete the good work. Preventing Grace may be likened to the impulse or contrition to give a good confession, and Co-operating Grace may be likened to the help given in making the good confession and doing the prescribed penance.
So what does this have to do with Mary? That singular gift which Mary received in being conceived without original sin so that the savior could be born of a woman free from sin, was given prior to her existence or at the moment of her conception – and so without her cooperation – for the sake of all mankind (Prevenient Grace). That grace led to another grace which inspired and enabled Mary to accept the will of God in all things (Concomitant Grace) and remain sinless. This is like grace begetting grace and confirms what Mary says in her canticle; that her soul magnifies the Lord (not to confuse Mary’s Immaculate Conception with the conception of our Lord).
Luke 1:46-5546And Mary said: My soul doth magnify the Lord.47And my spirit hath rejoiced in God my Saviour.48Because he hath regarded the humility of his handmaid; for behold from henceforth all generations shall call me blessed.49Because he that is mighty, hath done great things to me; and holy is his name.50And his mercy is from generation unto generations, to them that fear him.51He hath shewed might in his arm: he hath scattered the proud in the conceit of their heart.52He hath put down the mighty from their seat, and hath exalted the humble.53He hath filled the hungry with good things; and the rich he hath sent empty away.54He hath received Israel his servant, being mindful of his mercy:55As he spoke to our fathers, to Abraham and to his seed for ever.
In this canticle Mary recognizes that the Lord has ‘remembered’ His promise to Israel, and that Prevenient Grace of Mary’s Immaculate Conception is the grace that goes before her, beginning the fulfillment of God’s promise in rescuing the Church (Israel) from sin.
Modern Catholic Dictionary – John A. Hardon, S.J., pp119, 437