This is the fundamental leap of faith ex scriptura that most have a problem with (including the Orthodox churches, according to Kolokotronis) - being filled with grace does make one blameless and washes away your sins, but it does not imply that you were in that state since birth.
So, you are setting up a straw man. You presume that we teach that St. Stephen was not in fact without sin as he was martyred.
I did no such thing, and if it was implied, then accept my apologies! Rather, I wanted to use the example of Stephen being filled with grace as a case where a clearly NOT-sinless man was also filled with grace, as Mary. Meaning that being filled with grace does NOT confer with it a state of being without sin since conception. In effect, Stephen is a key example that shows the opposite of the conclusion made about Mary.
Our Lady is proclaimed by Archangel Gabriel already filled with grace. This is why the grammatical prefect tense is important: here is a young girl and she is said to be filled with grace already. This the scriptural basis not merely of her sinlessness but also of her immaculate conception: since she had been filled with grace prior to Archangel Gabriel talking to her, it is reasonable to think that she had been that way since the beginning of the life, since Sts Joachim and Anna concieved her.
Except that the root caritow means favored, NOT sinless. In fact, there is no foundational claim for caritow to mean sinless - either canonically or implicitly. Mary was favored, and found favor in the eyes of God, but she was NOT sinless. Perhaps you could try to interpet the Bible in that way, but it is far from obvious or unequivocal.
Furthermore, one needs to look no further than Romans 3:23 - ALL have sinned and fall short of the Glory of God. Not all but Mary, but ALL. Man's sinful nature is complete and inherent since Adam and Eve.
Of course. This is why I drew the distinction between St. Stephen and Our Lady. I agree that both were filled with grace at a certain time in their lives, and therefore their sin was washed away: Mary was filled with grace even prior to the Annunciation and St. Stephen -- at his electin as deacon and subsequent martyrdom. (And I was filled with grace this morning along with a couple hundred other people). The inference that Mary was filled with grace since her conception is made because otherwise we have to presume some oher moment in her life when that outpouring of grace had occurred, and the Scripture is silent on that. In the case of St. Stephen it is clear from context that he received the grace because of his faith just like he also received the blessing of martyrdom. Since he was an adult convert like all the deacons and bishops of the Early Church, we cannot presume anything of his life prior to the ordination. However, if you wish to believe, on the strength of the passage in Acts 6, that St. Stephen was sinless all his life, that would be a reasonable, especially for a Bible Alone Christian, interpretation. It just does not happen to come from the Catholic Holy Tradition.
I agree that if we were go by scripture alone, we would have some competing interpretations of Luke 1:28. But we don't go by scripture alone: the Church received the deposit of faith from the Holy Apostles in its entirety, which in due course produced both the New Testament Scripture and the infallible interpretation thereof. My task here as a Catholic is not to deduce the Catholic doctrine from the Scripture but rather explain the Scripture in the light of the doctrine received by the Church from the Holy Spirit directly.
the root caritow means favored, NOT sinless
The root is charis (χαρις), grace. "Favor" is an translation suitable in non-theological contexts; do a parallel search of "grace" in your preferred translation in the New Testament and you will always or nearly always find the Greek original say "χαρις" or an inflection thereof. "κεχαριτωμενη" means something like "engraced". The only reason Prtoestant translations use "favor" is to downplay the significance of Archangel's choice of words. Of course it does not mean "sinless"; sinlessness is an inference, not the text itself.
Romans 3:23
But Noah and, of course, Christ are described in the Scripture as free from sin. We know of sinlessness of others. For example, what sin did the babies massacred by Herod commit? Would not reason compel us to think that a two year old baby cannot sin?
Further look at the context of Romans 3. Not only have "all" sinned, they also do not seek God, none of them is righteous and in fact all are quick to shed blood. It is clear that St. Paul speaks in generalities in that passage. Indeed, the entire passage is a paraphrase of Psalm 14(13). But the next Psalm speaks of righteousless: "He that walketh without blemish, and worketh justice: He that speaketh truth in his heart, who hath not used deceit in his tongue: Nor hath done evil to his neighbour", etc. Romans 3 is to be taken in the same way the relevant psalms are written, as a contrast between the sinful and the righteous, but not as a prooftext of universal sinfulness.