A convert (proselyte) fully accepted Judaism by performing ceremonies like baptism, sacrifice, and circumcision (see Acts 13:43).
A convert differed from a "God-fearer." A God-fearer was a person who accepted some Jewish beliefs, especially monotheism (belief in one God), without fully converting to Judaism (see Luke 7:4–5; Acts 10:2). To win a single convert likely means persuading God-fearers to fully convert by becoming circumcised (compare Galatians 2:3, 14; 5:2–12; 6:12–13).
of hell: The Greek word here is Gehenna. It was a valley outside Jerusalem used as a place of burning; See study note on Matthew 5:29.