I admire sincerity. I am drawn to people who are earnest. My wife has told me that one of my traits that attracted her to me was my sincerity.
I am not exactly sure why I find honesty and openness so engaging, and really...intriguing. I could speculate on why I have a preference for this, but I know enough about psychology to know that I can't really know why I like what I like. I don't have access to the part of my brain that determines things like that. The best we can do is create a model as to why we and others might have the preferences we do. Our common error is to think we actually discovered why when we have only come up with a reasonable speculation. An excellent article on how we don't have access to what motivates most of our behaviors is "Telling more than we know: Verbal reports on mental processes" by Nisbett, R. E. & Wilson, T. D. (1977) in the Psychological Review, Vol 84-3, pages 231-244.
Back to the topic of sincerity. I have found many people, members and non-members, who are very sincere. I respect that. I think that honesty with ourselves is one of the ingredients of learning things as they really are. But, I think all of us come to conclusions too quickly and with too much certainty. All we really have are data points, data points which are open to many different interpretations. Data points do not become evidence until one organizes them in support for their hypothesis. But, just because data points can be shown to support one hypothesis does not mean that there is not a different hypothesis out there that the data supports equally well or better. Example: a defense attorney may use the same evidence that the prosecutor uses to reach an entirely different conclusion. What I am getting at is that if your hypothesis is that God exists, there are data points that can be used to support it, but, if your hypothesis is that God does not exist, there will be an alternative explanation for the data.
The key in determining which hypothesis is more plausible or reasonable is in asking yourself which hypothesis fits the data best. In other words, how hard is it to explain away the data (how many unlikely scenarios have to combine to be able to fit the data to the hypothesis). To do this honestly, we must start on a level playing field, without judging the case before we hear the evidence. I guess I have a problem when we adopt one hypothesis and then put on blinders so that we quit gathering evidence and start to conveniently ignore other data. We think that we have learned enough or know all that we need to know. Honestly, I think "there are more things in heaven and earth...than are dreamt of in [our] philosophy" (Shakespeare in Hamlet).
Yet, I am humble enough to admit that I could be wrong. Perhaps sincereity with certainty (if acquired appropriately) is better than sincerity with openness.