"Belong, believe, behave?" or "believe, behave, belong"? Theologian Roger Olson has some thoughts on church membership:

What should a Christian church require for “belonging” in the full sense of church membership (including holding office and/or teaching)? At minimum a Christian church should require members to believe in (if not fully understand) the doctrines of the incarnation (deity and humanity of Christ) and Trinity. I would add also belief that all people stand in need of salvation which is by grace alone by faith and cannot be earned. In addition, members should affirm that Jesus is the risen Lord who left the tomb empty, lives forever more and will return in glory.
What should a church require for “behaving?” At minimum, a Christian church should require members to affirm repentance for sin and desire to live a Christ like life with the help of the Holy Spirit and the community of God’s people. The person should be baptized in the name of the Father, Son and Holy Spirit or in the name of Jesus Christ.
Anyone who cannot affirm those beliefs (even with mental reservations) and desire to live that kind of life should not be given full membership in any Christian church. However, that is not to say they cannot belong in some sense of the word, depending on whether they are perceived to be moving in the right direction. A person who flagrantly denies those beliefs and rejects repentance and living a Christ like life should not even be allowed to think he or she “belongs” (even as they are allowed to attend).