Is. 41:16 And you shall rejoice in the Lord;
                in the Holy One of Israel you shall glory.

Deut. 13:1 “If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, 2 and the sign or wonder that he tells you comes to pass, and if he says, ‘Let us go after other gods,’ which you have not known, ‘and let us serve them,’ 3 you shall not listen to the words of that prophet or that dreamer of dreams. For the Lord your God is testing you, to know whether you love the Lord your God with all your heart and with all your soul. 4 You shall walk after the Lord your God and fear him and keep his commandments and obey his voice, and you shall serve him and hold fast to him. 5 But that prophet or that dreamer of dreams shall be put to death, because he has taught rebellion against the Lord your God, who brought you out of the land of Egypt and redeemed you out of the house of slavery, to make you leave the way in which the Lord your God commanded you to walk. So you shall purge the evil* from your midst.

YHWH had redeemed the slaves of Egypt and called them to be separate people, his own possession. They were about the enter a land that was not in their possession and were called to take it as their inheritance from God. The Lord wanted Isaiah 41:16 to be true of this people. He wanted to display the power of his hand and the glory of his countenance to his creation that had rebelled against him.

Not only was there pressure from the outside to conform to the world around them, but also within their camp would arise wolves who would tempt them to serve other gods. Why? Surely these false prophets wanted the people to go after false gods, but what was God doing in allowing for these evil men to open their mouths and entice his chosen ones? He was testing them, to know whether they loved the Lord with all their heart, soul, mind, and strength (the first great commandment).

Quick Sanctification
What was the prescription? Quick sanctification. They were not supposed to squabble with these false prophets. They weren’t supposed to go through a troubleshooting map to see what they were supposed to do in relation to divine justice. They were supposed to stone this person immediately. This was someone’s son or daughter. Someone’s husband or wife. Someone’s Daddy or Mommy. Why such cruelty?

The better question lies in what you are attributing as cruel. Are you calling the stoning cruel? Be careful. The Lord’s justice is swift and pure and unsearchable. Yet this justice should be easily seen by a lover of God. God is the most precious thing in our lives. He has redeemed us from the curse of death and given us his Spirit abundantly. He has promised us eternal life and joy for the rest of our days (eternity) in his presence. Now the real cruelty lies in the fact that a false prophet is trying to draw people away from everlasting joy for the folly of bowing knees to metal and wood. These were meant for our our good but can be our demise. Our own foolishness that casts the image will be the very reason we forsake the cake for the sake of dung. We go to bite into our forkful of joy and found out it is excrement.

We must be killing sin or sin will be killing us – Romans 8:13 (per John Owen). We do not dally with the very thing that will kill us. Who would let their daughter play with a convicted child molester? It is folly! Put to death…purge the evil from your presence. Or the Lord will vomit you into the fires of eternal torture.