Many churches have official membership with guidelines on what is required to become a member. Some Christians object to formal membership because the true Church as the Body of Christ is made up of everyone who professes faith in Christ. There is no specific place in Scripture where believers are commanded to become formal members of a local church. However, believers are instructed in the New Testament to fellowship with other believers, to participate in the mission of the Church, to emulate the faith of their leaders, and to submit to the authority of the leaders of their church (Rom 12:3–21; 1 Cor 12:12–31; 16:15–16; Eph 4:11–16; 1 Thess 5:12; 1 Tim 5:17–19; Heb 13:7, 17; 1 Pet 5:1–5). Membership is a way for believers to formally identify a specific local church as the one they have aligned with to meet those aspects of Christian living.
Churches have different requirements for membership, but the basic requirements tend to be that you profess your faith in Christ, that you have been baptized (or you will be baptized before being accepted as a member), and that no one in the church community has any reason to object to your membership. If someone does object, that does not automatically disqualify you from membership. Generally, the church leadership will discuss the objection privately with the person making the objection to determine its validity and relevance to your membership.
For churches organized in a way that requires members to vote on certain issues of church business (e.g., a decision to hire a new staff member or appoint an elder or deacon), having an official membership is necessary for determining who is permitted to vote. Certain volunteer leadership positions in the church are also only open to members (e.g., elder or deacon). Even for churches where a board of elders or a denominational hierarchy (rather than the members themselves) makes decisions related to the operation of the church, membership is still a way for believers to formally indicate their association with a local body of believers. When prospective members sign a church membership agreement (sometimes called a “church covenant”), they are making a promise to others in the church to be part of their church community.