If They Are Not Citizens. People cross U.S. borders illegally and call themselves Americans, this is fraudulent. A person cannot legally claim membership in any organization with out first being accepted as a member. Membership in Gods kingdom cannot be assumed. You could not become a Christian in the Biblical church that Jesus founded without baptism.
Most Christian denominations use one or more of the above in considering church membership.
Many accept only a verbal acceptance of statements like I accept Jesus Christ as my personal savior, commonly referred to as a profession of faith or acceptance of the alter call. Protestant membership requirements are extremely broad and few have any membership requirements. A person that came to know Christianity late in life says I would like to call myself a Christian. Their are qualifications. There is a considerable difference between calling oneself a Christian and being a confirmed member of The Church of Jesus Christ or another denomination.