(Please read the Scripture passage before the homily.)
This is Paul’s only preaching to a Christian audience. The context is that Paul’s missionary work is coming to an end. He looks back into the past, acknowledges the present and looks into the future. The past gives us example; the future brings difficulties.
The enemy is like wild animals, which we get rid of as we fill in an area with humans. Roaring lions, ravenous wolves, sneaky rattle snakes and other dangers try to prevent human habitation. As the human population grows, the wildlife tends to die out, not without resistance.
As the good news of Christ starts to dwell in the land, the wild animals of error and sin will fight back. This is why Paul calls us to vigilance. We do not know the future with its possible dangers, challenges and opportunities, so we must take precautions against the boogey things that can stalk, maraud, persecute and harm us.