"I know that after I leave, savage wolves will come in among you and will not spare the flock. Even from your own number men will arise and distort the truth in order to draw away disciples after them. So be on your guard! Remember that for three years I never stopped warning each of you night and day with tears." - Acts 20:29-31
We have a neighborhood full of squirrels. Usually they are cute little creatures who scurry up and down the trees. On one occasion though, we had a rogue squirrel that chased people, jumped onto their backs and would bite them. Someone eventually got hold of the little guy and put an end to the spree of terror. While this little "nut case" is gone and things are back to normal, we never know what may suddenly jump up, so we are always on our guard.
Paul was far more concerned about "savagery of the soul" than "terror from the trees". He knew that people would come into the church with ulterior motives and lead believers astray. Some would come "into" the assembly and some would jump up from "within". His words of caution directed the Ephesians back to three long years of consistent teaching and warning from God's Word. He called on them to remember and be on their guard.
The church seems to be a quiet neighborhood where we should feel spiritually safe. But, there will always be those who will sneak in without warning and "distort the truth". We need to be well versed in God's Word and always alert to those who "will not spare the flock". Read the Word daily, study faithfully, listen carefully and be on your guard.