Can the severely mentally-disabled knowingly sin?

Can people with extreme mental disabilities and handicaps knowingly sin? If they are not able to knowingly sin, is their salvation in heaven guaranteed? Is this a blessing a parent of a severely disabled child can carry in his heart that, no matter what, this child will one day be in heaven?

Two requirements for the commission of personal sin are the abilities to know and to choose. If a person has mental disabilities that incapacitate his ability to understand his action and/or to choose to commit that action, then he cannot sin. If he is baptized, which washes away the original sin to which even the mentally disabled are subject, then he is absolutely assured of heaven.

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