'The Law and the Prophets'
Amos 3:7 Surely the Lord God does nothing, unless He reveals His secret to His servants the prophets.
Before God carries out any plan or will, He will send His prophets to prophesy or proclaim His coming events.
So God also sent His prophets to admonish and guide the Israelites from their sins and blindness. At times He gave His instructions, commandments and will for them to observe.
2 Chr 20:20b ..Jehoshaphat stood and said, "Hear me, O Judah and you inhabitants of Jerusalem: Believe in the Lord your God, and you shall be established; believe His prophets, and you shall prosper."
When the Jews believed His prophets, they were blessed by prosperity, peace and security. Even when they taken as captives, they will enjoy prosperity and peace in their foster nations just as in the case of Joseph, Daniel, Mordecai, Nehemiah and many other Jews.
Peace in Believing the Prophets
Jer 29:7, 10-11 And seek the peace of the city where I have caused you to be carried away captive, and pray to the Lord for it; for in its peace you will have peace... For thus says the Lord: After seventy years are completed at Babylon, I will visit you and perform My good word toward you, and cause you to return to this place. For I know the thoughts that I think toward you, says the Lord, thoughts of peace and not of evil, to give you a future and a hope.
Even during God's judgment, He still told the Israelites to settle down in Babylon and to live in peace by believing His prophets for God will always love men created in His image. He wanted them to live in peace with hope and a good future.
But beware of false prophets and false dreams which could destroy them. (Jer 29:8-9)
John the Baptist Ministry
Mal 4:5-6 "Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord. And he will turn the hearts of the fathers to the children, and the hearts of the children to their fathers, lest I come and strike the earth with a curse."
Before the end of the Old covenant when Jews were no longer be His people (great and dreadful day of the Lord), the Lord sent John the Baptist (as Elijah) to bring the Israelites to repentance for there were so much of hypocrisy then, that even parents and children did not trust one another.
John the Baptist was also sent to prepare the Jews to come back to the Lord by keeping the Mosiac Law so that they would be ready to believe Jesus as their Messiah.
Luke 1:16-17, 76-77 And *he (John the Baptist) will turn many of the children of Israel to the Lord their God*. He will also go before Him in the spirit and power of Elijah, 'to turn the hearts of the fathers to the children,' and the disobedient to the wisdom of the just, to make ready a people prepared for the Lord..... And you, child, will be called the prophet of the Highest; For you will go before the face of the Lord to prepare His ways, to give knowledge of salvation to His people by the remission of their sins
John preached Kingdom Gospel
Mat 3:1-2 In those days John the Baptist came preaching in the wilderness of Judea, and saying," Repent, for the kingdom of heaven is at hand!"
God sent John the Baptist to preach repentance and the kingdom gospel.
Jesus to fulfill the Prophets
For our Lord Jesus Christ to fulfill the Prophets, He must first believe John the Baptist and accept God's will and plan for the Jews by being baptized by John.