What does Isaiah 1:9 mean?
The Lord's response toward those who continue in ongoing sinful rebellion against Him is to utterly wipe them from the face of the earth. The classic example of this used throughout the Bible is the story of the sinful people of the towns of Sodom and Gomorrah in Abraham's day (Genesis 18:16–19:28).In addition to the scene of attempted homosexual rape of two angels by a mob described in Genesis, God held other sins against Sodom's people: "She and her daughters had pride, excess of food, and prosperous ease, but did not aid the poor and needy. They were haughty and did an abomination before me. So I removed them, when I saw it" (Ezekiel 16:49–50).
As Peter wrote in 2 Peter 2:6, God rightly condemned Sodom and Gomorrah to extinction for these sins. Isaiah adds in this verse that the same fate would have come on Judah if the Lord had not left a few survivors from the coming destruction of Israel. Isaiah speaks in the present tense about something that will come on Judah later. This is likely referring to the invasion by the Babylonians in 586 BC
Paul quotes this verse in Romans 9:29, describing as Isaiah does that God, in His grace and mercy, allowed a remnant of Israelites to survive then and another remnant later to be saved by faith in Jesus.