Not everybody who knew God knew about the body of Christ.
Since this body belongs to Christ, men didn’t know about it before Christ manifested himself in flesh. You can’t find the teaching of the body of Christ from Genesis to Malachi, but some people in those books knew God.
For example, God’s friend Abraham(Jam 2:23), Moses who had talked to God on Mount Sinai; they knew God while they knew nothing about the body of Christ. King David got a promise from God, but he didn’t get any clue about the body of Christ.
Not everybody who knew Jesus knew about the body of Christ.
The body of Christ is all the believers who trust salvation came from Christ’s death and resurrection. Since the body of Christ is built on the foundation of Christ’s resurrection, people can’t know it or become it before Christ rose again. Therefore, You can’t find the body of Christ in Matthew, Mark, Luke, and John which ended right away with Jesus rising from death; although some people in those books knew Jesus in person. For example:
“And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead. And they kept that saying with themselves, questioning one with another what the rising from the dead should mean.” — Mark 9:31-32
12 disciples followed Jesus and knew him in person, but they didn’t know why Jesus had to die or how he could come alive again. Therefore, they have no understanding of the body which was built on Christ’s death and resurrection.
Not everybody who knew the Holy Spirit knew the body of Christ.
“For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.” — 1 Cor 12:13
The only way to become the body of Christ is to get baptized by the Holy Spirit.
We can know from the old testament that the Holy Spirit worked on people by dwelling in the anointed people.
In Acts chapter 2, Peter and the other disciples were filled with the Holy Spirit at Pentecost, then they baptized the believers with water. Peter and the other disciples experienced the Holy Spirit by being filled with it, but they were not baptized by the Holy Spirit. They knew water baptism and the Holy Spirit, but not the Holy Spirit’s baptism or the body of Christ. You can’t find the teaching of the body of Christ in Acts chapter 2.
The body of Christ was a mystery hidden since the world began. Before it was revealed, nobody could know about it.
The revelation of the mystery was given to the apostle Paul. In his books alone(Romans to Philemon), the church today can know the body of Christ.
“祂用启示使我知道福音的奥秘；（正如我以前略略写过的。你们念了，就能晓得我深知 基督的奥秘。）……这奥秘就是外邦人在 基督里，藉着福音，得以同为後嗣，同为一体，同蒙应许。”——以弗所书3:3-6
“How that by revelation he made known unto me the mystery; (as I wrote afore in few words, Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)… That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel…” — Eph 3:3-6