the greatest OT prophet

RC Sproul in What is Baptism? on John the Baptist:

"Who was the greatest prophet in the Old Testament?" Some say Elijah; some say Isaiah; others insist on Jeremiah. Finally I say, "No, the greatest prophet in the Old Testament was John the Baptist." We sometimes forget that while we read about John the Baptist in the New Testament, he lived before Jesus inaugurated the new covenant in the upper room on the night of His betrayal. So the economy of the old covenant extended from the beginning in the garden of Eden to the time of the Last Supper. Thus, John the Baptist belonged to the period of the Old Testament, and Jesus said of him, "Among those born of women there has arisen no one greater than John the Baptist" (Matt. 11:11).

While John was the greatest Old Testament prophet, it was his task to announce the end of the period of Old Testament redemptive history, for the kingdom of God was about to break through. In the Old Testament, the arrival of the kingdom of God was an ambiguous future event. But John began his message with a radical note of urgency. He cried out, "Repent, for the kingdom of God is at hand" (Matt. 3:2). He was saying that the kingdom of God was not in the distant future, it was about to arrive.

John used two metaphors to illustrate the urgency of the hour. First, he said, "Even now the axe is laid at the root of the trees" (Matt. 3:10a). It wasn't as if the woodsman had just gone out into the forest and started to chip away at the bark of a tree, but he still had to swing his ax another thousand times before he could bring it down. Rather, the woodsman had already cut down to the very heart and core of the tree. John was saying that with one more blow from the ax, the tree would come down.

Second, John said, "His winnowing fork is in his hand, and he will clear his threshing floor and gather his wheat into the barn, but the chaff he will burn with unquenchable fire" (Matt. 3:12). The winnowing fork was a tool used by grain farmers to separate the wheat from the chaff. After the grain was threshed, that is, the seeds were separated from the husks, the farmer would use a large fork to toss piles of seed into the air so that the wind would blow away the lighter chaff, the final small pieces of the husks. The chaff would blow away, but the heavier seeds would fall back into the pile. John was saying that the farmer was not just thinking about separating the wheat from the chaff, nor was he walking to the barn to get his winnowing fork. Instead, the farmer's winnowing fork was in his hand and he was about to begin the final step in the processing of his harvest. The moment of separation, the crisis moment that would divide the good wheat from the useless and undesirable chaff, was about to happen. John was saying, "Israel, your King is about to come, the Messiah is at hand, and you're not ready."