Heresy just means something contrary to the Bible. The thing is that there are many Biblically viable doctrines that aren’t necessarily true. Part of the reason for vastly different Biblical interpretation is not only because the Bible says many different things, but rather because we speculate as to why those things are said.
Some common examples are whether or not water baptism is required and why, pre/post-trib rapture, the Trinity, and many many more. Heresy can be difficult to define in my opinion.
Like someone said, from someone’s perspective everyone is a heretic.