It is important to distinguish these two.  Every believer in the Lord Jesus is in “the” Assembly (Church)  Eph. 1:22, 23.

But in “an” Assembly there is a “within” and a “without” 1 Cor. 5:12, 13.  This believer was put out of “an” Assembly though still in “the” Assembly.

The maintaining of the “unity of the Spirit” requires diligence  (Eph. 4:3).  The power of the Holy Spirit is needed to carry out faithfully this responsibility.

To use our position of being perfect in Christ (Heb. 10:14) as permission to allow moral or doctrinal sin in “an” Assembly, is a most serious evil.  (Equally so for a believer to allow sin in his, her life).

Everyone is involved in the evil if it is allowed in “an” Assembly.  2 Cor. 7:11.  That Assembly ceases to have the character of Christ’s body altogether and ceases to be “an” Assembly of God.  To deny this is also evil.