In Matthew 8, Pastor Armstrong discusses how Jesus healing the leper was considered to be a miracle only capable by the Messiah. But in the Old Testament (2 Kings 5:10), we read of Elisha giving instruction for Naman to wash himself in the Jordan River and thus be healed of leprosy. So as Jesus was not the first to heal a leper how can the story in Matthew be considered a Messianic miracle?
As Pastor Armstrong explains in the teaching, the book of Leviticus contains an entire chapter (Leviticus 14) devoted to how a Jew healed of leprosy must respond to the healing. After centuries, the rabbis noticed that the rituals required by Leviticus 14 had never been used in all the history of Israel because no Jew had ever been healed of leprosy. This caused the rabbis to assume (correctly) that this miracle could only be done by the Messiah when He arrived. So the healing of a leper was a mark of the Messiah.
Naaman, the General, was not a Jew (he was Syrian), so he was not under the Law of Israel and therefore he was not required to follow the rituals found in Leviticus 14 after being healed. So his healing (though a miracle) did not serve as a Messianic Miracle because it never involved Leviticus 14.
So the Messianic miracle was not healing anyone of leprosy. It was healing a Jew of leprosy because that was the only way for Leviticus 14 to apply. Jesus Himself testifies that no Jew was ever healed of leprosy by the prophet:
Luke 4:27 “And there were many lepers in Israel in the time of Elisha the prophet; and none of them was cleansed, but only Naaman the Syrian.”
When Jesus did heal lepers, He required they go to the priests in fulfillment of Leviticus 14 so that the proof of the Messianic Miracle could be seen:
Luke 17:11 While He was on the way to Jerusalem, He was passing between Samaria and Galilee.
Luke 17:12 As He entered a village, ten leprous men who stood at a distance met Him;
Luke 17:13 and they raised their voices, saying, “Jesus, Master, have mercy on us!”
Luke 17:14 When He saw them, He said to them, “Go and show yourselves to the priests.” And as they were going, they were cleansed.
Finally, Jesus affirms that healing a leper was proof He was the Messiah when he answers John's disciples in Luke:
Luke 7:20 When the men came to Him, they said, “John the Baptist has sent us to You, to ask, ‘Are You the Expected One, or do we look for someone else?’”
Luke 7:21 At that very time He cured many people of diseases and afflictions and evil spirits; and He gave sight to many who were blind.
Luke 7:22 And He answered and said to them, “Go and report to John what you have seen and heard: the BLIND RECEIVE SIGHT, the lame walk, the lepers are cleansed, and the deaf hear, the dead are raised up, the POOR HAVE THE GOSPEL PREACHED TO THEM.
For more information on the Messianic Miracles, please read: What is the unforgivable sin?