I would like to make a distinction. Idiot and liar are insults (well, rarely, a lie can be proven, but not likely on an Internet forum). Heretic has a technical meaning, and when used as such should not be taken as insult. For example, if you profess to be Christian but deny the Virgin Birth of Christ, you are objectively a heretic.
More narrowly, the Catholic Church defines as heresy denials of certain Catholic doctrines, even if other Christian denominations disagree with them. For example, some Christians believe that The Virgin Mary did not remain virgin after the birth of Christ. This makes their belief on that score objectively heretical.
There is also a distinction between holding a belief that is heretical and being oneself a heretic. This is because in order to be called a heretic one has to have a full understanding of the correct doctrine. An ignorant person cannot be a heretic. So, there is in fact a dose of respect that comes with the term.
I don't know if other confessions use the word at all, but this is how Catholics use it. Please, do not think that if you are described as a heretic someone is trying to insult you. All that is typically meant by the epithet is that your belief does not conform to the teaching of the Catholic Church.
Another word with a potential to be taken as an insult is "anathema", or its English and Latin cousin, "excommunication". This is used to describe the same thing: that the anathemized belief or the excommunicated person is in contradiction to the Catholic (or Orthodox) belief system. It does not mean that the anathemized person is condemned to hell, or even that he is forever barred from communion. The purpose of excommunication is to call the person's attention to this unfortunate fact and invite him to correct his views and come back to communion.
I am not sure if confessions other than the Catholic and the Orthodox use these terms in the precise technical sense, but this is how we use them, and we mean no personal offense when we do.
The Missouri Synod of the Lutheran Church tends to use the term "heretic" only in reference to member of the ELCA.
I have to second annalex's comment, RM. Orthodoxy uses heretic and heresy in almost the exact way he has described Roman Catholic usage, the only difference being that for us a heretic is a heretic is a heretic whether or not they realize they are a heretic. To state that someone is a heretic or some belief heresy is merely a statement of theological fact within the contemplation of The Church. Neither are terms in any way synonomous with or equivalent to liar or idiot.
"Please, do not think that if you are described as a heretic someone is trying to insult you. All that is typically meant by the epithet is that your belief does not conform to the teaching of the Catholic Church."
True.
Back at the Academy, we in the Catholic Choir referred to our Protestant counterparts as the Heretic Choir, but we did so with the utmost of Christian charity...
To call such a statement false, incomplete, inaccurate, misinformed, etc. would not be “making it personal.”