What does Isaiah 6:3 mean?
The moment Isaiah describes is one of complete wonder, terror, and awe. The Lord has caused Isaiah to see Himself, seated on a throne (Isaiah 6:1). Behind or above the Lord stand an unnamed number of angelic beings called "seraphim." (Isaiah 6:2)Perhaps resembling something like fiery serpents or dragons, Isaiah describes these six-winged beings as calling out to each other in voices that shake the building. They are declaring the holiness of their Lord.
In the culture of this time, words were repeated to give them emphasis. They are saying, in essence, that the Lord of hosts is holy times three. Nothing in or about Him is not holy. They identify Him as the Lord of hosts, meaning that He holds authority over heaven's angelic beings, including themselves. He is their commander and ruler in all things.
What does it mean that the Lord is "holy?" The word means "set apart" and implies something unique and pure. In God's case, He is the absolute standard of perfected holiness. He is completely unto Himself and dependent on nothing else for existence, purpose, or character. In truth, then, "holy" is whatever and whoever God is. His nature and existence defines the entire concept of "holy."
To a lesser extent, God provides a way for things to become holy in the sense that they are set apart to become of and like Him. God's "holy people" are the ones set apart for His service. Believers are called to this ongoing transformation to become more like God (Romans 12:2). This is referred to as sanctification.
The seraphim are calling out that the world is filled with God's glory. Isaiah stands in this moment in awe of the glory of God inside the temple. The seraphim insist that His glory cannot be contained inside the temple. It is everywhere He is, and He is everywhere. The glory of God is inescapable (Romans 1:20).