I think the main problem you are having with understanding the Catholic teaching on original sin is that the use of the word “sin” is misleading. You are imaging that original sin is a “thing”, when in fact it is an absence - an absence of divine life, that is. It is not the guilt of Adam’s sin that is transmitted to all mankind, but the absence of divine life that was caused by the Fall. At baptism, this dark, empty space in the soul is removed by being infused with sanctifying grace, thus “removing” the mark of original sin from our souls.
More:
Q Why are innocent babies born with original sin?
A The “sin” in “original sin” is not the same as the “sin” that we confess in the Sacrament of Reconciliation.
Innocent babies remain “innocent” inasmuch as they are not “guilty” of anything. Original sin is not a “thing”; in a certain sense it is nothing where there should be something — and that’s the “sin”! God gave Adam and Eve the immense gift of divine life to pass it on to all of us. By their personal sin, they lost it and, as it were, couldn’t pass it on. **So we haven’t got the gift of divine life we ought to have, and so neither do we have the pledge of eternal life with God. This *deprivation ***is what we call “original sin.” I recommend reading the whole section on the Fall in the Catechism, par. 385-410.
Q I have heard it said that due to original sin, man continues to sin. However, if at baptism we are cleansed of original sin, shouldn’t that take away our propensity to sin? Also, what is the difference between our Blessed Mother, who was born without original sin, and a newborn who has just been baptized? Are they both free of original sin in the same way?
A Baptism frees us from
original sin, the absence of the divine life in us, but not from its “side-effects,” including what Catholic tradition has called “concupiscence” — which is, well, a propensity to sin. Since the Virgin Mary was preserved from original sin to begin with, Christian tradition has understood that she remained free from the inclination to sin also.
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**Q: *Why do religions baptize infants? I think the Bible is clear that we will be judged ourselves, not by something we are born with (cf. Ezek. 18:20). Several other passages indicate that one is born innocent but, at some later point, becomes guilty by choosing sin (cf. Rom. 7:9–11; Ezek. 28:15; Is. 59:1–2). ***
A: It’s quite true that babies have no personal guilt and are not guilty of personal sin. That is why repentance is not necessary for them at baptism. They have nothing to repent of. They inherited from Adam and Eve not sin but the consequence of sin, which is a lack of sanctifying grace or spiritual life. We call this
original sin, but it is really the consequence of the original sin that we are referring to.
Adam and Eve were created in a state of original grace or spiritual life. As long as they remained in God’s friendship, that original grace remained. But as soon as they sinned, they lost this precious gift, without which we cannot enjoy friendship with God or eternal life in heaven.
Adam and Eve could not pass on to their children a spiritual gift they no longer had. Through the sacrament of baptism, sanctifying grace is restored to the soul. This is why babies are baptized. The sacrament restores sanctifying grace to the soul, makes them children of God, and enables them to inherit heaven.
catholic.com/thisrock/2004/0401qq.asp