What is the meaning of this verse? God's people had turned away from their devotion to the true and living God and turned their devotion to false gods, idols. Isaiah, being moved by the Holy Spirit, spoke to God's people ("Ye are my witnesses") to remind them that there is only one true and living God, the LORD. There was no God before Him and there will be no God formed after Him.
Many people today reject the true and living God and worship false gods. Some even desire to be gods, which is the lie of the devil in Genesis 3 and the reason why Lucifer was ultimately thrown from heaven in Isaiah 14.