A.T. Robertson Commentary Galatians 4:30

"Howbeit what saith the scripture? Cast out the handmaid and her son: for the son of the handmaid shall not inherit with the son of the freewoman." — Galatians 4:30 (ASV)

Cast out (εκβαλε). Second aorist active imperative of εκβαλλω. Quotation from Ge 21:10 (Sarah to Abraham) and confirmed in 21:12 by God's command to Abraham. Paul gives allegorical warning thus to the persecuting Jews and Judaizers.

Shall not inherit (ου μη κληρονομησε). Strong negative (ου μη and future indicative). "The law and the gospel cannot co-exist. The law must disappear before the gospel" (Lightfoot). See 3:18,29 for the word "inherit."