So, in the New Testament sense of the word, “heresy” was the creation of a division, a sect, a faction, or a party. For this reason, the author of Acts uses the word to describe the different sects within Judaism (Acts 5:17; 15:5; 24:5; 14; 26:5; 28:22).
“Heresy” involved the dividing of a local assembly, not the rightness or wrongness of what the dividing party believed.
It’s true, of course, that a heresy could be created by someone pushing a false teaching on a local assembly, causing it to divide. Peter alludes to this when he warns that false teachers will secretly come into the church and introduce damnable heresies (2 Peter 2:1).