To answer your good question, let’s begin by reading Isaiah 66:15-18, “For, behold, the LORD will come with fire, and with his chariots like a whirlwind, to render his anger with fury, and his rebuke with flames of fire. For by fire and by his sword will the LORD plead with all flesh: and the slain of the LORD shall be many. They that sanctify themselves, and purify themselves in the gardens behind one tree in the midst, eating swine’s flesh, and the abomination, and the mouse, shall be consumed together, saith the LORD. For I know their works and their thoughts: it shall come, that I will gather all nations and tongues; and they shall come, and see my glory.” 

The word ‘behold’ is used many times in the Bible.  It is used to cause us to focus our attention on what is being presented to us.  What we see in these few verses certainly need our undivided attention for the Lord is speaking of His impending judgment against those who have hated and abused His people.  In this judgment, the world will clearly see that the Lord has committed Himself to bless His people and to punish His enemies.  In taking vengeance upon those that have misused His people, it is said the Lord will “come with fire”, which is a common representation of the wrath of God.  We read of that earlier in Psalm 50:3 which says, “Our God shall come, and shall not keep silence: a fire shall devour before him, and it shall be very tempestuous round about him.”  It is with fire and a sword that the Lord shall literally EXECUTE JUDGMENT against the enemies of His people.  While the KJV says the Lord will ‘plead’ with all flesh, the literal meaning is that He will judge all men. 

The sword referenced in verse 16, I believe, refers to God’s righteous judgment.  We read of this sword in Deuteronomy 32:41 which says, “If I whet my glittering sword, and mine hand take hold on judgment; I will render vengeance to mine enemies, and will reward them that hate me.”  So, it is with fire (the wrath of God), and the sword (the righteous judgment of God), that the Lord will come to execute judgment upon the guilty. 

These people who are the enemies of the Lord and His people are said to “sanctify themselves, and purify themselves.”  They carry on the pretense of being spiritual people, but at the same time they are “eating swine’s flesh”, which was abominable to the Lord and forbidden for His people as we read in Leviticus 11:7-8, “And the swine, though he divide the hoof, and be clovenfooted, yet he cheweth not the cud; he is unclean to you. Of their flesh shall ye not eat, and their carcase shall ye not touch; they are unclean to you.”  So, these people sought to give the appearance of being spiritual, but their lives manifested their deceitfulness.  Besides, the Lord said in verse 18, “For I know their works and their thoughts.”  Proverbs 24:12 states, “If thou sayest, Behold, we knew it not; doth not he that pondereth the heart consider it? and he that keepeth thy soul, doth not he know it? and shall not he render to every man according to his works?”  How true it is that no man can hide from God.  We cannot disguise our thoughts, our desires, our intentions, etc. from God.  The Lord Jesus stated as much when He said in Matthew 15:8, “This people draweth nigh unto me with their mouth, and honoureth me with their lips; but their heart is far from me.”  1 Samuel 16:7 tells us clearly that, “…the Lord looketh upon the heart.” 

When will this judgment take place?  This will happen when the Lord returns at the end of the seven-year Tribulation period as we read in 2 Thessalonians 1:7-10, “And to you who are troubled rest with us, when the Lord Jesus shall be revealed from heaven with his mighty angels, IN FLAMING FIRE TAKING VENGEANCE on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be PUNISHED WITH EVERLASTING DESTRUCTION from the presence of the Lord, and from the glory of his power; When he shall come to be glorified in his saints, and to be admired in all them that believe (because our testimony among you was believed) in that day.”  (CC)  (727.6)