I showed yesterday what the "old and new" was that Jesus spoke about in the reading from last Sunday. Today I want to apply that principle to us in our theology. When our Lord gave authority to the Apostles to develop the Church, He showed them how to keep the balance of the growth of the New Covenant, while learning from the Old Covenant which came before. They did this perfectly (by the inspiration of the Holy Ghost) and that is why the Church is still here, and heresies fade away and die. The Catholic Church is, in fact, the perfect outgrowth of all that the Old Testament said was coming with the Messiah.
For us today, we must recognize that there are going to be people who seek not both the "old and new" as the Apostles taught us, but rather some imbalanced twist on this idea. In the first century, the Pharisees wanted "old and old"; in other words, they did not want to have anything change the status quo they created. A few years after the Resurrection of Christ, a few other heresies came along that wanted "new and new"; they sought to dump all in the Old Testament and taught that Jesus created a whole new religion unrelated to what God gave to Israel.
In our day, we may not have Pharisees or other first century heretics (at least they do not use those names!), but we still have people who make the same errors. Those who want to live in a fantasy world that was created in 1965 are seeking "new and new". I read stories on a regular basis that show many Catholics (including clergy) want to jettison everything in the Church that came before Vatican II ("the absolutest bestest, most fantastical council the Church ever had ever, at all"). This can be seen best in the rejection of the older liturgies and the promotion of a new morality.
There are also, however, those who want to have "old and old". These are the traditionalists who condemn everything after 1965. They do not just want to maintain the faithful Catholic Tradition that has been handed down for two thousand years. They also want to ignore the fact that the Church is growing to maturity and needs to deal with the storms of the world's confusion (cf. Ephesians 4:11-16). By their understanding of the faith, the Mass would still be in Greek (realize: there were those 16 centuries ago who thought translating the Mass into Latin was kind of liberal!).
The Church grows, and is balanced in her growth. She realizes that what Jesus taught yesterday is useful for today, and that we can always come to understand things better if we follow the pattern of learning that the Apostles gave to us. They truly were scribes who were properly trained by their Master, and they were able to bring out of their treasures wonderful gifts that were "new and old". This is how the Church survives that attacks of the world: by using all that Christ has given us in accord with the "faith once delivered to the saints".