The argument used here is a simple one, and a strong one: one head, one body. No one argues that: it makes sense, and deals with biology the way we understand it.
But not every saved person is part of the body of Christ. Let’s examine what Paul says about this body of Christ:
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The Facts About the Body of Christ
Romans 4
4 For as we have many members in one
body, and all members have not the same
office: 5 So we, being many, are one body in
Christ, and every one members one of