It’s clear that in the days of the apostles, it was the universal practice to receive believers into the visible church. It’s possible that a true believer, because of some unusual circumstances, may fail to unite with the church. One may, for instance, believe in Christ and die before receiving baptism, or joining a local church. But such instances are exceptional. The Scriptural rule is that while membership is not a prerequisite for salvation, it is a necessary consequence of salvation.