Harvest & Threshing = Resurrection & Glorification
Threshing = Separating the Wheat (the inner, spiritual body) from the Chaff (the outer mortal-natural body)
Just as wheat is harvested and then threshed, the body is resurrected then changed (glorified).
At harvest, the grain is suddenly cut free from its earthly connection to lifeless soil.
At threshing, the outer hull (the hollow shell husk, chaff) is suddenly removed to reveal the real substance beneath, the grain. The grain had been changed, separated from its husk of unprofitable outer chaff.
At resurrection, the saint is suddenly cut free from his earthly bonds to death (the grave).
At glorification, the mortal-natural body (the outer-man, biological, physical body of flesh, blood & bone) is suddenly removed (symbolised by circumcision) to reveal the real substance beneath, the spiritual body that had been born at his "Born-Again" experience. His body is changed, separated from its unprofitable outer-man, the body of flesh, the mortal-natural body.
Matthew 3:12 & Luke 3:17 ~preached by John the Baptist around 27AD
"I indeed baptize you with water unto repentance, but He who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire. 12 His winnowing fan is in His hand, and He will thoroughly clean out His threshing floor, and gather His wheat into the barn; but He will burn up the chaff with unquenchable fire."