In Chapter 12, Calpurnia takes Jem and Scout to First Purchase African M.E. for Sunday service. When Zeebo, Calpurnia's son, begins to leads the congregation in singing hymns, Scout notices that there are no hymnals in the church. Scout mentions, "This was too much for me," and doesn't know how they are going to sing without any hymnals (Lee 74). Zeebo proceeds to sing each line, while the congregation repeats the line after him. Scout says, "Line for line, voices followed in simple harmony until the hymn ended in a melancholy murmur" (Lee 75). The technique of repeating lines is called "linin' " and it is necessary because the majority of the congregation is illiterate. The two reasons that First Purchase African M.E. does not have hymnals are because hardly anybody would be able to read from them, and because hymnals would be an unnecessary added expensive to an already poor church.
In Chapter 12, we learn that it is because hardly any of the congregation can read: Zeebo (Calpurnia's son) leads the hymns, singing each line and having everyone repeat it back to him.
The other reason, of course, is simply because the church is so poor: and Lee undoubtedly presents it as more spiritual because of its lack of material wealth. From the book:
First Purchase was unceiled and unpainted within. Along its walls unlighted kerosene lamps hung on brass brackets; pine benches served as pews. Behind the rough oak pulpit a faded pink silk banner proclaimed ‘God is love’, the church's only decoration except a rotogravure print of Hunt’s The Light of the World. There was no sign of piano, organ, hymn-books, church programmes-the familiar ecclesiastical impedimenta we saw every Sunday.