Part of this reading really struck me this morning as I was reading the interpretation provided by the Message. That was: "A will, ratified by God, is not annulled by an addendum attached 430 years later, thereby negating the promise of the will." Maybe it's because I work in the legal field and, in particular, have been doing more estate planning services recently, but through this reading, I received a crystal clear picture of Paul's meaning.
In this case, Paul was simply stating that God's message through the ancient laws was not "undone" by the arrival of Jesus as our Savior. Instead, Jesus amended the promise by making it even easier to take full advantage of the abundant life that God was promising. Before Jesus, it was nearly impossible to acheive everlasting life due to the abundance of the sin of Man. We put a wrench in the ability of God to delivery on His promise. God tried to deliver on His promise by adding laws for us to live by. That still didn't work to fully reconcile Man to God's purpose. So, He sent Jesus.
Sometimes, we put impossible conditions on ourselves or those situations we are involved in or on people we love. For example, we may say to ourselves that we will spend more time with our spouse. But, instead of doing it, we add a condition, such as "...if he would get home earlier from work" or "if she would just listen to me". Those conditions build until it is impossible for us to act on our original purpose. So, we need to "amend" our purpose not to necessarily change the conditions, but to make our original purpose work in spite of those conditions - "I will spend more time with my spouse no matter how much time he spends working". The condition is still there, since it is not something we can control, but we've worked backwards to insure that the original purpose is still met.
So our lesson today is to be aware of the initial promise and to make sure that we don't let those conditions get in the way of meeting that promise.