"He was not that Light, but was sent to bear witness of that Light" (John 1:8).
Perhaps the least understood and appreciated man in the Bible is the man who was the first Christian soul winner, missionary, martyr, disciple, and pastor, as well as the first to make disciples for Christ, to acknowledge His deity, to baptize converts to Christ, to be imprisoned for his faith, and to teach concerning Christ, as Christ later commanded in His great commission.
John the Baptist is often mistakenly called the last Old Testament prophet, but that was Malachi, 400 years earlier. John was actually the first Christian! He leaped in joyous recognition of Christ while still in his mother's womb (Luke 1:44); in fact, he uniquely (apart from Christ) was "filled with the Holy Ghost, even from his mother's womb" (Luke 1:15).
He was such a godly man that he was often mistaken for Christ, and John the apostle even had to stress that John the Baptist was not really that Light, but that God had sent him to bear witness of the true Light-Jesus Christ. Christ Himself testified that no greater man than John the Baptist (not even Noah, Abraham, Job, Moses, or Daniel!) had ever been born (Matthew 11:9-11).
In no sense was the message of John an "Old Testament" message. He preached the deity of Christ, redemption by His sacrificial death, and salvation only through faith in Him (John 1:34,29; 3:36). He "began" the gospel message (Mark 1:1-2) and baptized and taught disciples whom he then directed to Christ. Their baptism was "Christian baptism." In fact, John had baptized Christ Himself! Finally, when he had "[made] ready a people prepared for the Lord" (Luke 1:17), he was imprisoned by Herod and executed because of his faithful witness for Christ. He had given the "knowledge of salvation unto His people" (Luke 1:77). HMM