Sin and error have a way of pervading the heart and mind. As we dabble in things and thoughts that can steal our focus from the only One worthy of devotion, so the stirring of His Spirit wanes. A little leaven leavens the whole lump. And one day we wake up wondering why His anointing seems distant and unfamiliar.
It comes down to momentary choices: What do I want more, God or this thing, this relationship, this substance? If the choice is difficult, then it is possibly something that has taken a portion of our heart designed for our Creator. God wants every part of us. It isn't that He is selfish, He just knows us perfectly. He knows what is absolutely best and what would be most fulfilling for our depths.
Jericho was completely destroyed because God is holy and He desires His people to be set-apart from things and thoughts that will only lead to bondage. Joshua explains, "And you, by all means abstain from the accursed things, lest you become accursed when you take of the accursed things, and make the camp of Israel a curse, and trouble it." (6:18) Yeast has everything to do with it.