ahiresis

In Acts this is the ordinary, unremarkable word for a named party inside Judaism. The Sadducees are one, the Pharisees are another, and a first-century Jewish reader would have heard no insult in it. These were groups within the same religion, reading the same scriptures, differing over the resurrection, the law, and how strictly it should be kept. Paul can even speak with some pride of having lived by the strictest of them.

The sting appears only when the label is aimed at a movement whose right to exist is in question. In Jerusalem and again in Rome, people pin the word on the followers of Jesus, and the lawyer prosecuting Paul uses it to make them sound like a breakaway group rather than the true heirs of Israel's hope. Paul takes the word without flinching and then quietly changes the terms: he worships the God of his ancestors, believes everything written in the law and the prophets, and calls the movement "the Way".

Behind the noun stands a verb meaning "to choose". A party of this kind is defined by what it has picked out and holds to, which is why one and the same word can name a respected school of thought in one sentence and a damaging split in the next. Everything hangs on whether the choice was a legitimate one.

Paul uses the word for something that goes wrong inside a congregation. At Corinth the church gathers for a shared meal and breaks into rival camps, the well-off eating ahead of the poor. In Galatians the word sits in a list of ruinous behavior, beside quarrelling, jealousy, and self-seeking ambition. The problem is not that people hold views. It is that they have organized around their own preference and set themselves against everyone else.

It helps to see how close it sits to the words around it:

• Paul first names the tears in the fabric (σχίσμα), then this word, which is a step further along: a rift has hardened into a party with a membership.
• In the Galatians list it follows the word for standing apart and stirring up disagreement (διχοστασία), which names the behavior, while this word names the group that behavior produces.

One line among these texts has been argued over for centuries. Peter warns of teachers who will smuggle in destructive ones of these, and readers have taken that either as the splinter groups such teachers gather or as the ruinous positions they push. Either way, this is the doorway through which the word passed into later Christian use, where it hardened into the English "heresy": a named doctrinal error. That later meaning is not yet what the New Testament writers had in hand.