How does Steinbeck use religious imagery connected with the burial of Rose of Sharon’s dead baby in order to further the novel’s political agenda?

Uncle John floats the child downstream. The symbolism is strong, it evokes the story of Moses baby being sent down the Nile. Later Moses would deliver his people from slavery. Rose takes on the role of a Mary-figure by giving the sick and the weak nurturing life.

The political "agenda" surrounds the socialist ideology of the weak breaking their chains of bondage from an oppressive force and then living a life of sharing in a socialist utopia.