Some professing Christians deny the necessity of church membership. Others join a church for unsubstantial reasons or leave a church for trivial, often selfish reasons. Many remain members of apostatizing churches because of family or traditional ties. Some Christians find themselves in areas or countries where no true church exists or can be formed. They ask, sometimes in anguish, "What must we do?" Seemingly forgotten today is the truth that Jesus Christ institutes his catholic church in organized congregations that are clearly identified by objective marks. These are true churches, in distinction from false and apostatizing churches. In the form of letters to an inquiring European audience, this book addresses the issue of church membership in the twenty-first century. This instruction is applicable to all believers and is based on scripture, the Belgic Confession, and the important, but little known, controversy of John Calvin with the Nicodemites.