It was not unknown but it was unusual in the 1st cent. to use ‘body’ for a collection of people as Paul does for his important phrase ‘You are the body of Christ’ (e.g. 1 Cor. 12: 27). He was probably attracted to the idea because of the Eucharistic words; the oneness of the loaf (called Christ's body) shared by the congregation led naturally to the application of the title ‘Body of Christ’.
There has been disagreement about the interpretation of the phrase—is the Church metaphorically or really the Body of Christ? Does it resemble the Body of Christ? Or is it in fact, in some way (but plainly not literally), the same body that lived and died and was raised in Palestine? On the whole Protestants have preferred the metaphorical interpretation. Catholics hold that the Church is sacramentally or mystically united to Christ—the instrument by which the heavenly Lord continues in the world what he had begun: he is the Head, the Church is his Body. For Protestants the Church is not the extension of the Incarnation any more than it is the extension of Christ's atonement, which is once and for all. Head and Body are separate from each other. Protestants point out that monstrous errors have been perpetrated by ecclesiastics who have identified their decisions with those of Christ himself.
Nevertheless Head and Body are inseparable, necessary to each other. Paul on the Damascus road is reported (Acts 9: 4) to have heard Jesus accusing him ‘Saul, why are you persecuting me?’, when he was persecuting Christians: there was a very close relationship between Church and Christ, and the Body is more than a mere metaphor. Perhaps Body language should be explained as similar to the Bride and Bridegroom imagery of Eph. 5; both are necessary to each other but are not identical.