There have been many questions about the idea that all things are forgiveable. This comment by J, is a good example. I hope my response to J is helpful to all of you who are wrestling with this idea, and perhaps a bit fearful of what embracing might entail. You are right to recognize that its a big claim, but it it deeply rooted in Jewish tradition and wonderful things can happen when it is embraced judiciously.

Mr Hirschfield Sir : The necessarily universal forgivability of all acts deemed unforgivable by some must therefore entail, according to you, that they necessarily forgive even those who are about to massacre them en masse. This is a very Christian idea, is it not? Remember the Amish who forgave the killers of their children; …

J,
For starters, we should never assume that the values which a community holds most dearly might not be lived more fully in some other community. I have often found that the practices and insights of non-Jews hold up a powerful mirror of the things to which I aspire as a Jew. In fact, this kind of mirroring is often how different religious communities grow. But that is not the main thrust of your interesting observation, to which I want to respond.
I think that you, and many of the commenters here, are confusing the fact that all sins are forgiveable with the idea that such forgiveability demands that all people forgive all sins. From a Jewish perspective, nothing could be further from the truth. In fact, there may be times when it is entirely appropriate not to forgive a particular act. But that decision is a function of the one doing the forgiving, not the nature of the transgression.
I know this may be uncomfortable because it leaves all of the power with us — we can not hide behind the claim that “it’s simply too bad to forgive”. And since we all like to think of ourselves as basically forgiving people, it is uncomfortable to take responsibility for the decision. Such is life and the power of choice!
We can only forgive the transgressions committed against us, so I am not even sure what it means to forgive somebody else’s murderer, unless we are forgiving them for the pain which they caused us. But for the hurts which others have caused us, we can always make the decision to forgive without losing our integrity. In fact, I have found that sometimes, it is in those moments that we really find it.