In 1 Kings 22: 21-23, Spirit is saying I will tell lies. How could this be? Is it an evil spirit? Please explain?

The verses being referred to are an excerpt from the story that recounts the death of Ahab, one of the kings of Israel whom God punished for his wickedness. By themselves, the verses may appear a little confusing, but context is important here.

When this passage speaks of the Lord “decreeing evil” upon Ahab, it’s not saying that the Lord committed an evil action; “evil” here is meant in the sense of an “evil fate”–that is, doom. In other words, the Lord is decreeing judgment over Ahab. So all this happens in the context of the Lord declaring his judgment over Ahab’s wicked life. God accomplishes this by permitting Ahab to be slain in battle.

What is unique is the way in which God does it. He basically lets Ahab fool himself into going into battle. If you read the rest of the cycle of stories regarding Ahab, you’ll notice that this king is a very arrogant and selfish figure. God basically uses Ahab’s own hubris, his own pride, against him in order to bring about his downfall. Ahab is full of confidence that he will not fall in battle and he aspires to conquer the neighboring kingdom of Aram.

Note carefully what the verses being referenced say. A “spirit” came before the Lord and said it would deceive Israel’s prophets. The “spirit” being referred to here is not God’s own Spirit, the Holy Spirit–only “a spirit.” The implication is that this is an evil spirit (a “lying spirit”) which God permits to act. It’s a situation akin to the story at the beginning of the Book of Job, where the devil goes before God in his heavenly court and requests permission from God to tempt Job.

God knows full well of the evil nature of these spirits, but he permits these spirits to act in such a way because they still accomplish his will (which in the case of this story, is the judgment he makes on Ahab’s life). The Holy Spirit cannot and will not lie to us; he is the “Spirit of Truth” (John 16:13).