Some evangelical churches require a person to be baptised before being accepted into membership of their congregation. Does this practice have the support of Scripture?

What I see in Scripture is this: the gospel is proclaimed, those whose hearts have been touched, whose minds have been illuminated, believe the gospel and are baptised. The saved are then added to the universal body of Christ, the church  (Acts 2:38-41; 5:14). Paul preaches the gospel 'and many of the Corinthians who hear him believe and are baptised' (Acts 18:8). Like their 3,000 brethren on the Day of Pentecost the Corinthians are also added to the universal body of Christ. Once saved, they are in the church Jesus came to build. They are the living stones Jesus uses to build his church (1 Peter 2:5). Paul wrote to the Corinthians calling them 'the church of God in Corinth' and proceeds to identifiy them with the rest of the universal church, 'together with all those everywhere who call upon the name of the Lord Jesus Christ - their Lord and ours.' (1 Corinthians 1:2).

The Corinthians and all other believers had this in common: they believed the gospel and were baptised. Because they were in fellowship with God, they were in fellowship with one another. Fellowship existed among them; nothing more was required and nothing less was accepted. God’s children formed communities, congregations in their villages, towns and cities. As people believed and were baptised, they identified themselves with one of those communities and were accepted into fellowship. That's how it was in the early church.

A scenario: X has been attending a particular church for months and feels drawn to the Lord. He is encouraged to commit his life to Jesus. He does and is now regarded by this church as having been saved. X now wants to partake of the Lord's Supper but is told that he is excluded from the table of the Lord until such time as he is baptised. If X says he wants to be a member of the congregation he is told that this is not possible until he is baptised. The church believes that X is saved, he is part of the universal body of Christ, but the membership of the local church is closed to him until such time as he is baptised. Baptism, according to this church, is necessary for church membership. What they are saying is that people can be saved and receive the  Holy Spirit along with all the blessings that are found in Christ, and yet cannot be a member of a particular church without baptism. This practice is not to found in the Bible.

The apostolic practice, which we must hold to, was to proclaim the gospel and baptise, immediately, those who believed. Believe and be baptised was their practice. No one waited for months or years to be baptised, not if they believed. The apostles knew nothing of baptism for the purposes of church membership, though it is a common practice today.