Paul knew what that New Covenant entailed, even if it was not written down yet. The OT is not obsolete; its interpretation and practices are.
The interpretation and practices of the jews. That doesn't mean that there is NO validity to the old testament. The example I've been using it is that of the Lord's holy days. You seem to be of the mindset that since the jews observed them, or since they were first record in the books of the old testament, that there must be something wrong with them.
RE: "The OT is not obsolete; its interpretation and practices are."
Spoken like a true Constantinian Christian!
ONE OF THE MOST SIGNIFICANT DIFFERENCES BETWEEN JUDAISM AND CHRISTIANITY IS WHETHER OR NOT THE LAWS THAT GOD GAVE MOSES, TO TEACH TO THE CHILDREN OF ISRAEL, ARE STILL IN EFFECT TODAY. IN THIS SECTION I WILL BE DEMONSTRATING HOW MUCH IMPORTANCE GOD PUTS ON HIS LAWS AND HOW DIFFICULT IT IS (FOR ME) TO EVER THINK GOD WOULD NO LONGER WANT HIS LAWS FOLLOWED.
Clear explanation at source:
http://www.mindspring.com/~sjayg7/sample.htm