Dan,
Christ is at the head of BOTH the Kingdom churches, as well as the Body of Christ. Just because a believer in Christ was of the Kingdom churches which were started by the apostles, does not mean that they were not "in Christ". When the Lord Jesus "gathers all things that are in Christ", it is written that they will be things in heaven, where the Body of Christ has their eternal hope, and things in the earth, where the Kingdom of Heaven shall be.
Anything pertaining to Christ's followers in the churches are things "in Christ", whether they are Kingdom believers, or Body believers.
D |