Scripturally, only the membership in Christ is of eternal value, and that’s the only kind that you will find it mentioned explicitly throughout the New Testament (particularly in Paul’s writings) many times.
Membership in a religious organization can have potential value, as far as its realm of influence is concerned, and as long as membership requirements coincide with (and not supplant) the New Testament theology, comply with the New Testament model of servant leadership, and don’t overreach into a person’s freedom in Christ in a cult-like fashion.
If those basic principles are observed, then such membership can be a wonderful tool of advice, help, accountability, and growth. If they are not observed, it can become a tool of control, manipulation, and bondage that has a real potential in leading people away from God, which as we all know happened in the 2000 years of church history time and again.
Abraham Lincoln once remarked on this matter:
When any church will inscribe over its altar, as its sole qualification for membership, the Saviour’s condensed statement of the substance of both Law and Gospel, “Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and thy neighbor, as thyself,” that church will I join with all my heart and all my soul.
If it’s not formal membership, then what is the glue that binds the body of Christ in perfect unity? Paul mentions 2 binding agents in his epistles:
13 Bear with each other and forgive one another if any of you has a grievance against someone. Forgive as the Lord forgave you. 14 And over all these virtues put on love, which binds them all together in perfect unity.
Make every effort to keep the unity of the Spirit through the bond of peace.
Love binds our virtues together in a cohesive, holistic set which is then expressed through our character. That starts out working on an individual level, and proceeds to express itself corporately in groups of individuals of like character.
Peace, on the other hand, is the glue that binds us together and gives cohesiveness, integrity, and unity to our corporate expression.
These two binding agents make up the recipe for the glue that holds the body of Christ together, and makes is an authentic, tangible expression of the agape love of God through Christ to the world.