Are they Hebrews?: Paul's opponents viewed Jewish heritage as a qualification for ministry (see Philippians 3:5).
Are they Israelites?: An Israelite was part of the covenant community through circumcision (compare John 1:47; Romans 9:4). Paul had this mark too and was a Pharisee, a very devoted follower of the Torah (Acts 26:4–5; Romans 11:1; Philippians 3:5–6).
Are they descendants of Abraham?: This was another claim by Paul's opponents to God's favor based on ethnic identity. Elsewhere, Paul explains who can truly be called Israelites and descendants of Abraham (see Romans 2:28–29; Galatians 3:16, 26–29). Here, he ironically uses his opponents' flawed perspective to outdo them.