First Purchase is named after the first purchase the freed slaves made in Maycomb, the materials to build this church. However, the church is missing many of the accoutrements that Scout associates with her own Methodist Church. One of those things is hymn books. Consequently, we find out that the congregation engages in something called 'linin'' in order to sing their hymns. Zeebo, the local trash collector and Calpurnia's son, comes to the front of the congregation with the one and only hymnal we see in the church building. He begins by telling the congregation and the name and page number of the selection, something which is quite odd to Scout. He then begins by saying the first line, which the congregation then sings, ending with a low hum, which Zeebo then talks over with the next line. The low hum then breaks into song as the second line is sung, ending again in the low hum. And each line of the hymn is sung in this way.
Scout is utterly amazed at this technique. Not only with the accuracy of the words, but the harmonies the church members achieve without benefit of music. It is not until later, outside the church that she finds out that not does this keep the church from needing hymnals, but the church members couldn't read the hymnals if they had them. There are only about 4 members of the church who can read.
The first thing that freed slaves in the Maycomb area purchased with wages earned as free men was the land and materials needed for their Methodist church. Zeebo, the garbage man who we also saw pick up the rabid dog shot by Atticus, would read a line from a hymn and the congregation would repeat it. This was necessary because the church didn't have money for hymnals and, as Calpurnia explained, it wouldn't matter if they did because few members of the church could ready, anyway.
The First Purchase African M. E. got its name from what it was: the first thing that freed slaved purchased with money they'd earned with free labor. At that church, one man (Zeebo) sang a line, and then the others sang it back. This was because most in the church could not read.