Verses 18 and 19 discuss the enemies of the cross. To be forewarned is to be forearmed. In verse 17 Paul encouraged the Philippians to mark out those who are mature. The negative crowd we run with influences our values. They gain their identity from what others in their group think about them.
Teens are vulnerable to the crowd.
When they are by themselves they seem to be fine. When they run with the crowd that is another story. They are easily led. Group pressure sweeps them along in values they would not otherwise hold. If they get into the right crowd where they can see living, flesh and blood, examples of those who walk with the Lord, the impact will be for good.