Actually, it does not impact the doctrine of Original Sin any more so than the general exception to the No Salvation outside the Church does. I agree that the concept of Limbo is certainly one resolution to the necessity of Baptisim. However I think we need to keep in mind some basic points.
- Actual physical baptisim is not strictly speaking necessary. Under extreme circumstances, one can receive baptisim by sincere desire to receive baptisim.
- Infants are incapable of formulating that sincere desire.
- The Church however, allows that those who are not formal members of the Church because of ignorance but whom seek God with a sincere heart may be saved by being mystically linked to the Church.
Now on point 3. As far as I can tell, this path is at least technically open to all, even those without an explicit knowledge or belief in Christ and therefore baptised.
Now, we know that God is both Just and Merciful, more over, he is perfectly just and merciful. If the Church teaches that Invincible Ignorance does not prevent someone from being within the mystical church, it should certainly be the a legitimate defense for a child who dies prior to baptisim. It is certainly true that a child in infancy is incapable of being guilty for their ignorance. Likewise one could argue that they have an innate desire for God, even if they cannot articulate that desire. Therefore, it seems, to me anyway, that arguing that such infants are saved in the fullest sense does not argue against the doctrine of Original Sin, but rather argues for both God’s Justice and his Mercy.
Mind you this not mitigate the need to be baptized any more than desiring the eucharist mitigates the need to actually receive it. Actual reception of baptisim will strengthen us spiritually to resist the temptation of sin.
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Bill