To my mind, the key concept is the "probity in intention" part. In other words, sincerity is to be measured looking forward. It isn't really about how you feel or express yourself today. The idea is to follow through on what you say, and actually to perform on the promises you make, and actually to do the things to which you commit. Sincerity is an absolute requirement of any genuine politics.
It turns out that the most prevalent "image" found by a search for images related to the word "sincerity" is an image of a wedding dress.