The LORD promised that He would be the God of Abraham and his descendants. This is a promise that God kept, but His people, from the sound of the Gospel for today, appear to treat Abraham as their God more than they treat the LORD as their God. They claim that being a son of Abraham is their claim to the covenant, and that is all that’s needed.
We may look at this as foolishness, especially from our hindsight view of them, that they were ridiculing Jesus by argument from their own heritage. But how often do we do the same? I know you’ve seen them – the people who go to Church every Sunday, but leave right after Communion. Maybe you know them – the ones that went to church all the time, but stopped going because they believe they’ll get to Heaven because they’re "a good person". Maybe you are one of them – born into a Catholic family (a "cradle Catholic"), but don’t go to church since Confirmation/high school. Or maybe you’re on the other side – you go to Church, give money each weekend, but the commitment ends at Sunday, and you never get involved in any ministries.
All of these are examples of counting on our heritage – either heredity or customs – to get us to heaven. The problem is, we know of far too many cases of religious people with kids who fall away from the faith. There is no such thing as a "2nd-generation Catholic". Each of us must be born again. Going through the motions does not truly make us children of God and heirs to the kingdom. At best, it might eventually wear down our resistance over a long period of time, so that we actually start to really participate. At worst, it can make us "pretenders to the throne" – people who think they are religious and holy, so that they can look down at or criticize anyone and everyone else, fellow Catholic or not.
Are we claiming heaven as our because of our lineage? Would we recognize and accept God when he confronts us with our hypocrisy or our laziness? Do we even take the place of God, confronting others for "not following the faith" or "not being generous enough", pointing out the splinter in another’s eye while ignoring the log in our own?