Paul warns the Philippian believers—and us as well—to be on our guard against enemies of the faith. The ironic thing is that the enemies he focuses on are not unbelievers but rather religious people: the Judaizers. These Jews, like others prideful of their perceived religious and ethnic superiority, referred to gentiles as dogs. Paul called THEM dogs instead. And in spite all their religious zeal, he called them evil doers. Their mistake—their evil—was that they believed that righteousness was earned and attainable through ritual.
The reason passages dealing with that specific group in history are still relevant today is that their false teaching, in one form or another, has plagued the Church ever since. Legalism is a tricky issue, because true faith does change our behavior. The crucial difference between behavior arising from a legalistic understanding of the Gospel and a more Biblical one lies in motivation and manipulation. The motivation of a believer to do good should not come from a sense of obligation but desire. We are not earning God’s favor, but reacting to it. Manipulation of others through rules and rewards—power and control exerted over others—is the true appeal behind legalism. That has never been what the Gospel is about.
Here, religious legalism is seen to be fleshly. That is a clever bit of irony we get from Paul, because legalists and religious types like to claim that they are the only truly spiritual people. The opposite is true. Rather than outward, fleshly signs of goodness, it is the inner being that shows who we really are.