Heresy would be any doctrine that would damn men's souls to hell. An example would be a false gospel that adds or subtracts from the finished work of Christ, a Gospel which accomplishes complete salvation from foreknowledge to glorification to every believer. All converted will experience all phases of salvation mentioned in the golden chain of redemption in Romans 8:29-39
Adding or subtracting anything to this as also necessary for salvation is a false gospel.
Many do so out of ignorance though. As stated many times on here this is done by not distinguishing texts that describe a true believer from texts that prescribe what a person must do to be converted, saved &c. Perhaps God will bring them to the end of such error to the glorious truth of the Gospel and to full assurance.
There are those who preach other false gospels, little gods which is linked to prosperity gospel, name it and claim it, all implying that the Gospel is in its intent a message that God wants us to have material wealth &c. That is why Christ died for us according to their message, and the fact he died to save us from our sins is generally thrown in as a token, but is simply not the thrust of their false message at all: their message is one that is designed to appeal to the carnal lost nature and desires.
Then there is the false message that Christ is a moral example, and we must do and do and do to get in, and, that his righteousness is not imputed to us. Anyone preaching that message is a false teacher, and IMO, since they do not know the Gospel for obvious reasons, it is most likely they are yet lost in their sins. How can a person completely misunderstand the Gospel, work of Christ, what it accomplishes, mock it and those who believe, and yet be converted?
Forgive my frankness, but how stupid are these messages. Look at how they demean the work of Christ to the satisfaction of carnal desires as its accomplished objective.
Jude describes some of these preachers, and by all appearances they are not converted.