Does Church Membership Matter?

Back in the early days of the Church, there weren't any free-floating Christians, going around attending whatever church they felt like. We now that the Christians were counted, numerous times in the New Testament we hear, 5,000 became Christians today, 3,000 this day. They also kept track of them via baptism. Therefore, we can tell from the New Testament that Church Membership isn't something to be considered, it's a necessity.