Who is Isaiah writing about here in chapter 5? If we're not careful, we can quickly lift our head from our Bible, look around at our society and think with how bad things are right now in the United States that Isaiah probably encountered the same thing in his day. So God must have been talking about the unbelieving Gentiles. That is not the case. This is the wrong answer.
"Therefore the anger of the LORD was kindled against his people... (v. 25)." It was those who knew the truth who called darkness light and light darkness, etc. The so-called sons and daughters of God were the ones rebuked in Isaiah 5—not the pagan nations surrounding them. And Isaiah's call (coming up in Isaiah 6) was to preach a message of repentance to these bound to a holy, loving covenant to walk in faithfulness with their God.
Isaiah 5 demonstrates how the Lord speaks to His people. It is always in the context of the relationship He has with His children, and the relentless grace and kindness He has shown them. The Lord addresses His people's refusal to believe, the stubbornness of their hearts, and the flirtatious way in which they behave toward other so-called gods. (The words spoken to those on the outside the family of God are totally different; see Isaiah 10:5-19 and Isaiah 13:1-22 for examples of how God rebuked Assyria and Babylon.)
There's an expression that goes something like this: "You don't have to say anything, you're preaching to the choir!" Well, I don't know if you've noticed or not, but when the camera just panned across the choir, the members of that esteemed body looked pretty ridiculous: some were distracted and looking around, some were very concerned about their appearance, and some were asleep. This little cliche emphatically shuts down necessary reminders for sinful, forgetful people. God spends much of His time and effort in Scripture "preaching to the choir." Just like our passage here in Isaiah, it's critical because we all get sidetracked, we forget, and honestly we often just don't care anymore.
While there is still time and we have an audience with ears to hear, let us all encourage, challenge, rebuke and comfort "the choir." And let's remember the words of the apostle Paul: "Therefore let anyone who thinks that he stands take heed lest he fall" (1 Corinthians 10:12 ESV).