A.T. Robertson Commentary Acts 5:17

"But the high priest rose up, and all they that were with him (which is the sect of the Sadducees), and they were filled with jealousy," — Acts 5:17 (ASV)

Which is the sect of the Sadducees (η ουσα αιρεσις των Σαδδουκαιων). Literally, "the existing sect of the Sadducees" or "the sect which is of the Sadducees," η being the article, not the relative. Hαιρεσις means a choosing, from αιρεομα, to take for oneself, to choose, then an opinion chosen or tenet (possibly 2 Peter 2:1), then parties or factions (Galatians 5:20; 1 Corinthians 11:19; possibly 2 Peter 2:1). It is applied here to the Sadducees; to the Pharisees in Ac 15:5; 26:5; to the Christians in 24:5-14; 28:22. Already Luke has stated that the Sadducees started the persecution of Peter and John (Acts 4:1f.). Now it is extended to "the apostles" as a whole since Christianity has spread more rapidly in Jerusalem than before it began.