The dominant translation which ancient Christianity has given is very clear: the Byzantine tradition in the East and the medieval tradition in the West have seen in "kecharitomene," the indication of Mary's perfect holiness...
The verb utilized here by Luke (charitoun) is extremely rare in Greek. It is present only two times in the New Testament: in the text of Luke on the Annunciation (Luke 1:28). "kecharitomene," and in the Epistle to the Ephesians (Ephesians 1:6), "echaritosen." ... These verbs, then, effect a change of something in the person or the thing affected. Thus, the radical of the verb 'charitoo' being 'charis' (= grace), the idea which is expressed is that of a change brought about by grace. In addition the verb used by Luke is in the past participial form. "Kecharitomene" signifies then, in the person to whom the verb relates, that is, Mary, that the action of the grace of God has already brought about a change. It does not tell us how that came about. What is essential here is that it affirms that Mary has been transformed by the grace of God ...
The perfect passive participle is used by Luke to indicate that the transformation by grace has already taken place in Mary, well before the moment of the Annunciation.
In what then would this transformation of grace consist? According to the parallel text of the Letter to the Ephesians 1:6 the Christians have been 'transformed by grace' in the sense that 'according to the richness of His grace, they find redemption by his blood, the remission of sins.' (Ephesians 1:7). This grace, in reality, takes away sin. This is elucidating for our particular case. Mary is 'transformed by grace', because she has been sanctified by the grace of God. It is there, moreover, in the Church's tradition that we have the most customary translation. Sophronius of Jerusalem, for example, interprets the term ,full of grace, in this manner: 'No one has been fully sanctified as you ... ; no one has been purified in advance as you.' In addition, he takes from the total context that Mary had been 'transformed by the grace' of God in view of the task which she awaits, that of becoming the Mother of the Son of God, and to do so while remaining a virgin.
There we have the double announcement of the angel: as mother she brings to the world the Son of the Most High (v.33), but that will take place by the 'power of the Most High' (v.35), that is virginally. God had prepared Mary for this by inspiring in her the desire for virginity. This desire of Mary was then for her a result of her transformation by grace.
It is true that we do not find in the text of Luke evidence that Mary is "full of grace" from the first moment of her existence. But what in reality does the proclamation of the dogma of the Immaculate Conception say? Grace has preserved Mary of all sin and of all consequences of sin (concupiscence). This is also the biblical understanding of the concept of "grace." Grace takes away sin (Ephesians 1:6-7). If it is true that Mary was entirely transformed by the grace of God, that then means that God has preserved her from sin, "purified" her, and sanctified her....
As one can notice in the schema of the structured text the theme of "being full of grace" is continued in the first proclamation, "You have found grace with God"; then follows the substance of the announcement: Mary will become the mother of the Messiah. It is apparent that Mary was "full of grace" by God in view of this maternity, and even that she was prepared, by the grace of her virginity, for her own mission, that of being the virginal mother of the Savior.
