Acts 20
25 “And indeed, now I know that you all, among whom I have gone preaching the kingdom of God, will see my face no more. 26 Therefore I testify to you this day that I am innocent[e] of the blood of all men. 27 For I have not [f]shunned to declare to you the whole counsel of God. 28 Therefore take heed to yourselves and to all the flock, among which the Holy Spirit has made you overseers, to shepherd the church [g]of God which He purchased with His own blood. 29 For I know this, that after my departure savage wolves will come in among you, not sparing the flock. 30 Also from among yourselves men will rise up, speaking [h]perverse things, to draw away the disciples after themselves. 31 Therefore watch, and remember that for three years I did not cease to warn everyone night and day with tears.

Paul warns the elders that 2 things will happen:
Wolves will come in among them.
People within the group will rise up with perverse thoughts and ideas.

So the enemy is within, and the enemy is on the outside. Without the Lord there is no hope of us living a life of victory.
We must continue to work out our salvation with fear and trembling. We must live a life of love spoken about in Corinthians 13.

Using the Holy Spirit we can be more in tune with where the enemy is. Then we will know whether a brother in Christ is struggling, or if he is being used by the enemy. When you are in tune with the spirit you can also determine what is false teaching verse what is truth and you will not be lead astray.