What does it mean that we are members of Christ's body, flesh, and bones?

Being members of Christ’s body, flesh, and bones means believers are, in the spiritual realm, totally integrated with Christ Himself. At salvation they are placed in Christ, and He also dwells within them. This two-way union is pictured by marriage and called the great mystery concerning Christ and the church.

Les explains that Ephesians 5:30 is impossible to comprehend except by faith, but it is what the Book says: believers are members of His body, members of His flesh, and members of His bones. In the spiritual realm, this means we have become totally integrated with Christ Himself—we are in Him. Scripture also teaches the reverse: He is in us. Les calls this a two-way street. The moment a person is saved, he is placed in Christ, and Christ also comes and dwells within him. That is why Paul uses the marriage relationship. Verse 32 calls it a great mystery or secret, one of Paul’s words, now revealed to this apostle, never seen elsewhere in Scripture, not even in Israel’s relationship with God or Abraham and Moses with Jehovah. The context also shows Christ loved the church, gave Himself for it, sanctifies and cleanses it by the washing of water—not baptismal water but the water of the Word—so He might present it to Himself a glorious church without spot, wrinkle, or blemish.

Ephesians 5:30

For we are members of his body, of his flesh, and of his bones.

Why are Christians called members of Christ's body?

Christians are called members of Christ’s body because they have been united with Him and made one with Christ, just as husband and wife become one flesh. The church is His body, and believers are members of His flesh and bones, cleansed and set apart by the Word.