The sentence that states the central message you want to communicate is the

thesis statement
topic sentence
conclusion
hook

Thesis statement.

Explanation:

A thesis statement is a concise formulation or summary of the main idea, point, or topic that you wish to elaborate throughout any form of dissertation. It should, ideally, be expressed in a single sentence and this is recommended to be placed in the introductory section —depending on your rhetorical skills, your argumentative line, and academic traditions being followed, it is recommended to place it either at the middle of the introduction or at the end of it.  

the mending wall was written by robert frost

hasty generalization