Consider the verses quoted at the opening for a moment. They unquestionably accept the Old Testament as the absolute foundation for Christ Himself and all that follows in what we now call the New Testament. In Luke 24:44-45 Christ not only identifies the entire Old Testament in His description of its three basic categories (“Law of Moses”, “Prophets”, “Psalms”), but proceeds to open the minds of His followers to understand the entire body of Scripture in the Old Testament.
In the early church there is a significant span of years between Christ’s ascension into Heaven and the distribution of the letters and writings eventually to become known as the “New Testament.” During those years without a New Testament – and even beyond – one of the primary tools was the Old Testament. It was used by the Apostles – evidenced in particular by their extensive quotation of it throughout their letters – and by all those teaching/preaching the Gospel. When the Gospel is brought to Berea, for instance, it is examined and proved by the hearers of the message through their examination of the Old Testament:
“The brethren immediately sent Paul and Silas away by night to Berea, and when they arrived, they went into the synagogue of the Jews. Now these were more noble-minded than those in Thessalonica, for they received the word with great eagerness, examining the Scriptures daily to see whether these things were so. Therefore many of them believed, along with a number of prominent Greek women and men.”
– Acts 17:10-12
The most wonderful application of the Sermon of the Mount as presented in Matthew 5-7 is how Christ, quoting directly from the Old Testament, shows Himself to the be the fulfillment of the Old Testament as He now transitions to the New Testament, built on the sound foundation of the Old.