Brethren are brothers and sisters in Christ. United with God by the Spirit of holiness. Holiness of heart and conduct are the "fruit" of this blessed union made with Christ, the King of King's and Lord of Lord's. One with God and separate from the world, the flesh, and the Devil. Sin has no dominion over the Brethren of Christ because the Holy Spirit "sanctifies" these members of Christ's body from the power of sin. Becoming a Brother or Sister in Christ is why Christ is not ashamed of calling these personal members of His body..."Brethren".
Hebrews 2:11...For both he that sanctifieth and they who are sanctified are all of one: for which cause he is not ashamed to call them brethren,
Holy Brethren are "now", "on this side of Heaven". The Brethren are sanctified holy...thus eligible to partake in the Heavenly call. The Brethren acknowledge the enthronement of Christ...and remain subject to His authority and presence in their life.
Hebrews 3:1...Wherefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus