From Catholic Answers, Karl Keating:
Think about a child below the age of reason. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. If the child dies before ever committing an actual sin, because he isn’t mature enough to know what he is doing, what act of his brings him under their interpretation of Rom. 3:23? None, of course.
Paul’s comment to the Christians in Rome thus would seem to have one of two meanings. Despite the phrasing, it might be that it refers not to absolutely everyone, but just to the mass of mankind (which means young children and other special cases, like Mary, would be excluded without having to be singled out). If not that, then it would mean that everyone, without exception, is subject to Original Sin, which is true for a young child, for the unborn, even for Mary—but she, though due to be subject to it, was preserved from its stain.
It took a positive act of God to keep her from coming under its effects the way we have. We had the stain of Original Sin removed through baptism, which brings sanctifying grace to the soul (thus making the soul spiritually alive and capable of enjoying heaven) and makes the recipient a member of the Church. We might say that Mary received a very special kind of “baptism” at her conception, though, because she never contracted Original Sin, she enjoyed certain privileges we never can, such as entire avoidance of sin.
(Started at para. 9)
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