We’re told in scripture to deal with our anger. To not let the sun go down while we are still angry. Or else we will give place (or a foothold) to the devil. I’m sure there are many ways the devil can gain ground in our lives. He is/ they are crafty.
An easy way, but one we may be oblivious to, is when we don’t go out of our way to show love, even to those who dislike us. We remember from scripture that Jesus tells us to love our enemies, to do good to them, to pray for them. But we often can find ourselves holding back, and simply tolerating them, with a friendly politeness which can be a thin veneer over an anger we are suppressing. Or over a disregard for them. All that is wrong. And when we do that, we sin, and give the devil ground in our lives.
God lets the devil have his way in our lives when we do this, to teach us a lesson. Of course with restraints. But this ends up being a part of God’s loving discipline of us. An example of how God works in all things for the good of those who are in Jesus.
The best way to avoid giving the devil ground is to resist him with God’s armor provided for us in Jesus. And to seek to be sensitive to the Spirit and with reference to God’s word, as well as seeking to live in harmony with Christ’s Body, the church. And when we fail, to promptly confess our sins.
We in Jesus are in a battle against the world, the flesh and the devil. This in truth has been called the unholy trinity, although I hate to use that word in any way analogous to the Blessed Trinity. We do well to understand this, and to live with knowledge accordingly in Jesus. That we might be a blessing. To the glory and praise of God.