With this statement Peter is referring to the false prophets who existed during the time of the Old Testament. The people in question would thus be the people of Israel.1 Unlike true prophets, false prophets did not speak with divine authority (Deuteronomy 18:20; Jeremiah 14:14; Jeremiah 23:21, Jeremiah 23:32; Ezekiel 13:2–7), they conveyed a message of peace and security in contrast with judgment (Jeremiah 4:10; Jeremiah 6:14; Jeremiah 14:13, Jeremiah 14:15; Jeremiah 23:17; Jeremiah 27:9, Jeremiah 27:16–18; Ezekiel 13:10; Micah 3:5, Micah 3:11), and they were condemned to punishment by God (Jeremiah 14:15; Jeremiah 23:15; Jeremiah 28:16–17).2 By referring to the false prophets, Peter is drawing a typological connection between what happened in the past and what is happening in the present. Just like there were false prophets amongst God’s people in the past, we should not be surprised to find false teachers in the present.
1 But false prophets also arose among the people, just as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Master who bought them, bringing upon themselves swift destruction.