Stay alert! Watch out for your great enemy, the devil. He prowls around like a roaring lion, looking for someone to devour. 1 Peter 5: 8 NLT
Because of the context of Ephesians 4: 27, I would often associate the opportunity or foothold for the devil with anger, only. The Holy Spirit has made it clear to me that he (our enemy) seeks to "get in where he fits in". his prowling is not limited to anger. he will use your anger, your grief, your low self-esteem, as well as your triumphs. For your anger, he's got rage and violence. For grief, he has depression and suicidal thoughts. Low self-esteem can bring about envy, as well as promiscuity. Triumphs can cause us to be prideful.
Stay alert, watch out, don't give any opportunity are all calls to action. We can't passively watch the devil devour when we've been given weapons for battle.
Therefore, put on every piece of God’s armor so you will be able to resist the enemy in the time of evil. Then after the battle you will still be standing firm. Stand your ground, putting on the belt of truth and the body armor of God’s righteousness. For shoes, put on the peace that comes from the Good News so that you will be fully prepared. In addition to all of these, hold up the shield of faith to stop the fiery arrows of the devil. Put on salvation as your helmet, and take the sword of the Spirit, which is the word of God.
Pray in the Spirit at all times and on every occasion. Stay alert and be persistent in your prayers for all believers everywhere. Ephesians 6: 13-18 NLT
Don't give the enemy a foothold or any opportunity to run amuck in your life or the life of your brothers and sisters, pray.
