Was Jesus an Old Testament Prophet? —Which covenant did he preach?
Some people think the words of Jesus in Matthew, Mark, Luke, and John, belong to the Old Testament. They represent Jesus as the last Old Testament prophet. They say the New Testament was revealed later by Christ's apostles.
1 Jesus a Prophet Like Moses
We read that Jesus was "a prophet like Moses"(Acts 3:22-24,cf Deuteronomy 18:15-18). Does this mean that Jesus was an Old Testament prophet like Moses was? No —Moses typified Jesus Christ and Jesus was the New Testament antitype of Moses the Old Testament type. As the antitype of Moses, Jesus can be said to be like him. But since antitype is greater than type, Jesus is unlike Moses in certain attributes...
The Law came through Moses but grace and truth came through Christ (John 1:17-18).
Christ was the "spiritual rock" typified by Moses who gave the people physical water from a physical rock (1Corinthians 10:1-4).
Moses put a veil on his face because his was a ministry of condemnation. Christ's was a ministry of unveiled glory and liberty (2Corinthians 3:7-8,17).
Christ was faithful like Moses, but Moses was a servant of God as a testimony of those things that would be spoken later by Christ the Son of God (Hebrews 3:1-6).
After Moses spoke the commandments of God he cleansed the book and the people with animals' blood. Christ after he had spoken his message offered his own blood (Hebrews 9:11-12,19-20).
All of the above would lead us to expect Christ's teaching to be superior to Moses's teaching. Christ did, in fact, say on occasions, "You have heard that it was said in olden times... but I say to you..."(eg Matthew 5:21-48). It is inaccurate to say that here Jesus was giving a “better interpretation” of the law of Moses. In fact he was contradicting it (eg Matthew 5:31-32).
2 Jesus Establishes a New Time Zone
Jesus was going to end the Old Testament law and bring in a new covenant in his blood (Romans 10:4,Luke 22:20,Hebrews 8:6-7). Why would Jesus preach the old covenant that he was about to fulfil rather than preach the new covenant he was to mediate?
Moses was one of the prophets who spoke "in times past to the fathers", whereas Jesus "in these last days" was the one by whom the gospel was "first spoken"(Hebrews 1:1-2,Hebrews 2:3).
The Old Testament prophets were "until John" and "since that time" the gospel was preached. So Jesus was preaching his new covenant, the gospel. He was not preaching the Law (Luke 16:6,Luke 20:1).
3 Jesus Understood the Mystery
Here's a closing thought...
The Old Testament prophets spoke things that they did not fully understand even though they made careful enquiry and sought to know about Christ (1Peter 1:9-12) Would he keep it a secret?
The gospel is "the revelation of the mystery which has been kept secret for long ages past but now is manifested" (Romans 16:25-26). No good reason can be given as to why Jesus, who is the very manifestation of the mystery, would perpetuate the mystery in his own teaching.