It’s all plain and yet grand, curious yet noninvasive. This country has a long history of white people happening upon black musicians and screaming, “Eureka.” This movie just wants to show what its subjects would like to tell. This is to say that it’s all retrospect, too. Smith and Dorsey reminisce about their life stories and tell a history of gospel music. The filmmakers catch them just in time, to hear about this music from the people who, decades before, essentially invented and perfected it.

Smith and Dorsey are both long dead. Their lives were happy. Their lives were sad. Their lives, essentially, were lives. And all of that life is in their music, which, thanks to Dorsey’s innovation, weds the plaint, woe and human upside of the Negro spiritual with the rhythm of the blues. It was music meant for the church, but the old folks of the 1930s and ’40s found it lacking sufficient churchliness. To the elders, this new gospel was nothing but sin. They didn’t hear a happy marriage so much as a shotgun wedding.

In “Say Amen,” these two are the elders now. The centerpiece happens to be a tribute to Smith, whose pupils have come out to serenade her. A loving, gentle opening detail sets the stage for the afternoon, which the movie cuts in and out of. It’s just a D.J. in a booth promoting the festivities. But there’s something so loving in the knowing politeness of his pitch. Come out and see, he says, “such inspiring singers as the Delois Barrett Campbell and the Barrett Sisters from the Windy City of Chicago, Saint Louis’s own Miss Zella Jackson Price, and the favorite O’Neal Twins with the Interfaith Choir. Don’t miss it.” Each name is timed with a shot of an album cover — and this was an age when album covers could put across a lot. But, here, they’re coming attractions for the roof-raising being done in Smith’s honor. It’s a day of love but also of vocal athleticism. (Fittingly, the Interfaith Choir comes down the aisles in robes of sports-jersey red and white.)

The singing, though, is just one element in a larger ruminative, perceptive work. The film picks up on the chauvinism just beneath the surface of the church, and in picking up on that, it’s aware of the feminism clinging to it, like Velcro. Barrett Campbell is making breakfast for her husband, and they’re figuring their schedules, when he wonders how much longer he’s going to have to wait until their respective ministries can be one, instead of her running around doing the lord’s work with her sisters — ferociously, I might add. She makes a half-smile, lays a quick look on him and asks, after a pause, “Do you want eggs with your sausage?”

These kinds of casually loaded moments are also spilling out of this movie. So are some real questions about where gospel music, in the early 1980s, after disco and before “Thriller,” is headed and where it can go in a country that, as one of the O’Neals observes, is a “hit-music society.”