Each of these novels, Go Tell It on the Mountain and Their Eyes Were Watching God, feature protagonists who struggle to find their own way.
In each novel, the protagonist is born into a distinct culture with definite rules of behavior and definite belief systems. These belief systems are explored in the novels and, in part, explained as to the origination, development and general usefulness of the belief system within the culture.
However, the individuals who anchor these novels feel the need to question those belief systems (a tendency which leads directly to the exploration cited above) and to break away from them.
These are books about the personal-spiritual “place” where a profound need for spiritual truth overlaps conflicts with the notion of accepted, cultural truth. In order for the protagonists in these stories to believe at all in any truth, the truth must be discovered, not received.
Following this through-line, a number of thesis statements might come to mind.
Why not make one of your sub themes epiphany. The song is all about coming to a realization and spreading "the word", which is similarly explored in Their Eyes...