Jesus commissioned His disciples to be His witnesses in Jerusalem, Samaria and to the ends of the earth.
…you will receive power when the Holy Spirit has come upon you; and you shall be My witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth.” Acts 1: 8
After the ascension of Christ, and after the sending of the Spirit, the disciples began to fulfill the commission. In those very first days there was no New Testament; when the disciples taught about Jesus, they taught about Him from the Old Testament and demonstrated how Jesus fulfilled the prophecies in Scripture.
There is a great example of this methodology in the book of Acts, when Philip encounters an Ethiopian who is studying God’s word.
But an angel of the Lord spoke to Philip saying, “Get up and go south to the road that descends from Jerusalem to Gaza.” (This is a desert road.) So he got up and went; and there was an Ethiopian eunuch, a court official of Candace, queen of the Ethiopians, who was in charge of all her treasure; and he had come to Jerusalem to worship, and he was returning and sitting in his chariot, and was reading the prophet Isaiah. Then the Spirit said to Philip, “Go up and join this chariot.” Philip ran up and heard him reading Isaiah the prophet, and said, “Do you understand what you are reading?” And he said, “Well, how could I, unless someone guides me?” And he invited Philip to come up and sit with him. Now the passage of Scripture which he was reading was this: “He was led as a sheep to slaughter; and as a lamb before its shearer is silent, so He does not open His mouth. In humiliation His judgment was taken away; who will relate His generation? For His life is removed from the earth.” The eunuch answered Philip and said, “Please tell me, of whom does the prophet say this? Of himself or of someone else?” Then Philip opened his mouth, and beginning from this Scripture he preached Jesus to him. Acts 8: 26 – 35
Why did Philip need to explain the passage from Isaiah to the Ethiopian? Because when Isaiah wrote the passage, he did not write “this is about Jesus of Nazareth who will be born in about seven hundred years.” Only someone with an understanding of who Jesus is can interpret the life and work of Jesus and compare it to the text from Isaiah, to demonstrate to those listening how Jesus fulfilled the writing of the prophet.
Christians today suffer a problem opposite of the Ethiopian. The Ethiopian had the Old Testament, but he did not know of Jesus and had no New Testament to read. Christians today, if they read the Bible at all, tend to read the New Testament and know little of the Old Testament. They have no idea how the Jesus they believe in fits into the overall narrative of Scripture, and how He is the One the Old Testament said was coming. The power of the gospel witness of many contemporary Christians is reduced because it is historically ignorant and lacks depth.
So to anyone who is ignorant of the Old Testament, or to anyone who is familiar and gets great joy in reading prophecies about Christ, behold, the famous passage from Isaiah, which the Ethiopian was reading when he was encountered by Philip. This is an amazing picture of Jesus Christ over seven hundred years in advance of His incarnation. The gospel is clearly portrayed – that a man would die on behalf of sinners, and as a result they would be justified before God and have their sins forgiven. The entire passage is remarkable and ought to bolster confidence that Jesus is the Messiah of Scripture, for no one other than Jesus matches the description of the suffering servant prophesied by Isaiah.
Who has believed our message? And to whom has the arm of the LORD been revealed? For He grew up before Him like a tender shoot, and like a root out of parched ground; He has no stately form or majesty that we should look upon Him, nor appearance that we should be attracted to Him. He was despised and forsaken of men, a man of sorrows and acquainted with grief; and like one from whom men hide their face He was despised, and we did not esteem Him. Surely our griefs He Himself bore, and our sorrows He carried; yet we ourselves esteemed Him stricken, smitten of God, and afflicted. But He was pierced through for our transgressions, He was crushed for our iniquities; the chastening for our well-being fell upon Him. And by His scourging we are healed. All of us like sheep have gone astray, each of us has turned to his own way; but the LORD has caused the iniquity of us all to fall on Him. He was oppressed and He was afflicted, yet He did not open His mouth; like a lamb that is led to slaughter, and like a sheep that is silent before its shearers, so He did not open His mouth. By oppression and judgment He was taken away; and as for His generation, who considered that He was cut off out of the land of the living for the transgression of my people, to whom the stroke was due? His grave was assigned with wicked men, yet He was with a rich man in His death, because He had done no violence, nor was there any deceit in His mouth. But the LORD was pleased to crush Him, putting Him to grief; if He would render Himself as a guilt offering, He will see His offspring, He will prolong His days, and the good pleasure of the LORD will prosper in His hand. As a result of the anguish of His soul, He will see it and be satisfied; by His knowledge the Righteous One, My Servant, will justify the many, as He will bear their iniquities. Therefore, I will allot Him a portion with the great, and He will divide the booty with the strong; because He poured out Himself to death, and was numbered with the transgressors; yet He Himself bore the sin of many, and interceded for the transgressors.