But in that case...

On the face of it, the best defence for the Holy Father's apparent desire to admit the divorced and 'remarried' to Holy Communion, is his view that many (or even most) marriages are in fact null, as the couple do not fully understand and believe in what they are undertaking.  If that is the case, if the first marriage was never in fact a marriage, then a second union may be entered into, and there is no question of adultery and thus no breaking of Our Lord's teaching.

But let's follow that train of thought a bit further. If we allow the hypothesis (and in fact, for many reasons, I do not) that most marriages are null, due to ignorance, lack of proper intent and so forth, then we should also address the fact that most people receiving Holy Communion do so in a similar state: they are equally poorly formed, and therefore quite probably not discerning the Body of the Lord - and we know what St Paul has to say about that. Further, do we conclude that most baptisms are probably invalid too, for similar reasons? In which case, people should certainly not be presenting for Holy Communion: I don't think even Kasper is advocating Communion for the unbaptised (or at least, not yet...)

But I think the problem lies in setting the bar of understanding too high. Just as a child can understand enough worthily to receive Holy Communion, without a full sacramental theology; so a couple - any couple who are capable of such understanding - can understand what the marriage vows mean.

The Church's immemorial practice, for reasons that are clearly wise, prudent and just, has always been to assert the validity of the marriage bond, unless there is clear and substantial reason in a specific case to prove that it was not valid. If the Holy Father intends to reverse that, we really are in trouble.