United to Jesus
In several passages of the Bible, the phrase "body of Christ" clearly refers to the actual flesh and blood, physical, material body of Jesus which was crucified on the cross.
Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God. (Romans 7:4)
Christ's bodily sacrifice on the cross for sin.
The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? (1 Corinthians 10:16)
Christ's bodily sacrifice on the cross for sin. Christ's literal, bodily presence in the Eucharist.
But Christians are also the body of Christ.
Now ye are the body of Christ, and members in particular. (1 Corinthians 12:27)
Christians, joined mystically in some way to the Spirit of Christ through his sacrificial bodily death to redeem us from sin.
For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ. (Ephesians 4:12)
Christians, same as above.
Do these two uses of the phrase "body of Christ" mean two completely different things? one referring to Jesus' physical body, and the other referring to Christians as a group?
In my view there was a progression...
Thus, it is perfectly natural to refer to Christians as the body of Christ since we are speaking of their soul, not their body. Just as the human nature of Christ is the "body of Christ" so also those whose souls are indwelt with the Holy Spirit are likewise the "body of Christ."
I should mention: Married couple have a similar kind union. Their souls in the spiritual realm get tangled-up together upon taking their marriage vows of living together in love forever.