Why does the Apostle Paul refer to the church as the ‘body of Christ’? His turn of phrase is based on an ancient concept called ‘Corporate Solidarity’. It’s an assumption that what is true for the significant individual is also true for the group or tribe which he represents.
Here’s a biblical example:
Yes, he [Jacob] struggled with the Angel and prevailed; He wept, and sought favour from Him. He found Him in Bethel, And there He spoke to us… (Hosea 12:4 – emphasis mine)
The prophet Hosea is declaring that, in speaking to Jacob, God was speaking to all Israel, for Jacob is a corporate solidarity for all his descendants.
Messiah has become the corporate solidarity for all his followers. That’s what Paul means when he talks about being ‘in Christ’ (as opposed to remaining within Adam’s corporate solidarity).
Since we are ‘in Messiah’, he is our corporate solidarity and we (figuratively) constitute his ‘body’.