“‘Full of grace’, Mary is ‘the most excellent fruit of redemption’: from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life.” (CCC 508)
Luke 1:28 [RSV]: "And he came to her and said, 'Hail, O favored one, the Lord is with you!'"
[The RSVCE translates kecharitomene ("favored one" above) as "full of grace"]
Catholics believe that this verse is an indication of the sinlessness of Mary — itself the kernel of the more developed doctrine of the Immaculate Conception.
The great Baptist Greek scholar A.T. Robertson exhibits a Protestant perspective, but is objective and fair-minded, in commenting on this verse as follows:
“Highly favoured” (kecharitomene). Perfect passive participle of charitoo and means endowed with grace (charis), enriched with grace as in Ephesians. 1:6, . . . The Vulgate gratiae plena “is right, if it means 'full of grace which thou hast received'; wrong, if it means 'full of grace which thou hast to bestow'” (Plummer).
(Word Pictures in the New Testament, Nashville: Broadman Press, 1930, six volumes, Vol. II, 13)
Kecharitomene has to do with God’s grace, as it is derived from the Greek root, charis (literally, “grace”). Hence, it is translated as grace 129 out of 150 times in the King James Version.
Presbyterian Greek scholar Marvin R. Vincent noted that the literal meaning was “endued with grace” (Word Studies in the New Testament, Grand Rapids, Michigan: Eerdmans, 1946, four volumes, from 1887 edition: New York: Charles Scribner’s Sons; Vol. I, 259).
Likewise, well-known Protestant linguist W. E. Vine, defines it as “to endue with Divine favour or grace” (An Expository Dictionary of New Testament Words, Old Tappan, New Jersey: Fleming H. Revell Co., four volumes-in-one edition, 1940, Vol. II, 171).
The angel is here, in effect, giving Mary a new name (full of grace), as if he were addressing Abraham as full of faith, or Solomon full of wisdom (characteristics which typified them).
Charis (grace) often means favor, it’s true, but it can also refer to a state. The latter is how Catholics usually think of grace: or more specifically, as a power or ability which God grants in order to overcome sin (and this is how we interpret Luke 1:28). This sense is a biblical one, as well.
For St. Paul, grace (charis) is the antithesis and vanquisher of sin (RSV):
Romans 5:20-21 Law came in, to increase the trespass; but where sin increased, grace abounded all the more, so that, as sin reigned in death, grace also might reign through righteousness to eternal life through Jesus Christ our Lord.
Romans 6:14 For sin will have no dominion over you, since you are not under law but under grace.
Romans 5:17 If, because of one man's trespass, death reigned through that one man, much more will those who receive the abundance of grace and the free gift of righteousness reign in life through the one man Jesus Christ. (cf. 2 Cor 1:12; 12:9; 2 Tim 1:9)
We are saved, of course, by grace, and grace alone:
Acts 15:11 But we believe that we shall be saved through the grace of the Lord Jesus, just as they will.
Ephesians 2:5. . . (by grace you have been saved),
Ephesians 2:8 For by grace you have been saved through faith; . . . (cf. Rom 3:24; 11:5; Titus 2:11; 3:7)
All of the above instances of “grace” in English are translations of the Greek charis, the root of the word used by an angel in Luke 1:28 to describe Mary: kecharitomene. From the above we learn two things, and they are biblically certain:
1. Grace saves us.
2. Grace gives us the power to be holy and righteous and without sin.
Therefore, for a person to be full of grace is to both be saved and to be exceptionally, completely holy. Thus we might re-apply the above two propositions as follows:
1. To be full of the grace which saves is to surely be saved.
2. To be full of the grace which gives us the power to be holy and righteous and without sin, is to be fully without sin, by that same grace.
Or, we could make the following deductive argument, with premises (#1 and #2) derived directly from Scripture:
1. The Bible teaches that we are saved by God's grace.
2. The Bible teaches that we need God's grace to live a holy life, above sin.
3. To be “full of” God's grace, then, is to be saved.
4. Therefore, Mary is saved.
5. To be “full of” God's grace is also to be so holy that one is sinless.
6. Therefore, Mary is sinless.
The only logical way out of this would be to deny one of the two premises, and hold that either (1) grace doesn't save, or that (2) grace isn't that power enabling one to be sinless and holy.
Another possible quibble might be about when God applied this grace to Mary. We know she had it as a young woman, at the Annunciation. Catholics believe that God gave her the grace at her conception so as to avoid the original sin which she inevitably would have inherited, being human, but for God's preventive grace, which saved her from falling into the pit of sin by avoidance rather than rescue, after she had fallen in.
In a very simple sense, the Immaculate Conception is God giving Mary the grace to be as sinless and innocent as Eve originally was, a thing quite fitting and not at all strange or implausible for one chosen to bear the Lord God in her own body.
It would be strange for a Protestant to underplay grace, given their constant emphasis on grace alone for salvation (with which we Catholics fully agree — we merely deny the tenet of faith alone, as contrary to the clear teaching of James and Paul). If grace saves, then to be full of grace is to not only be saved, but without sin, according to biblical principles and Protestant beliefs concerning sanctification. For no one can have more grace than to be “full” of grace.