Ill defined? How is the Scriptural definition “ill defined”?
1 Corinthians 12:27 Now you are the body of Christ and individually members of it.
And Paul was talking to the Corinthian church here, not the one at Rome.
1 Peter 2:4-5 As you come to him, a living stone rejected by men but in the sight of God chosen and precious, you yourselves like living stones are being built up as a spiritual house, to be a holy priesthood, to offer spiritual sacrifices acceptable to God through Jesus Christ.
Ephesians 2:19-22 So then you are no longer strangers and aliens, but you are fellow citizens with the saints and members of the household of God, built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, in whom the whole structure, being joined together, grows into a holy temple in the Lord. In him you also are being built together into a dwelling place for God by the Spirit.
Looks like Peter and Paul clearly recognized that the church, the body of Christ, is comprised of believers.
Does that include believers in false doctrines?
Not a rhetorical question.