Previously, Paul alluded to his own pursuit of Christ-likeness. Now he very directly calls the church to follow his example and that of others who live the same way. We are to imitate and keep our eyes on such examples. He gives us two reasons why: 1) many live for this world (their influence can drag us down!), and 2) our citizenship is in heaven (our hope is there, not on earth!).
Many live for this world (18-19): We need to follow the right examples. This is especially true when many around us are living for this world, and we can get sucked into their earthly mindset. Paul sees this influence as antagonistic to the cross of Christ. Thus, he calls these worldly people enemies of the cross. Now, their enmity isn't an attack on the theology or the history of the cross. This isn't a matter of re-interpreting the crucifixion or denying its historicity. Their attack is against the heavenly hope that Christ won for His people with His death. Their lifestyle devalues heaven and elevates the earthly life. They live a purely secular life where only what's here and now matters. And we must never adopt their worldview. So Paul identifies them for the flawed people that they are. First, they are hell-bound. To follow them is to follow them to hell. Second, they serve their own appetites as their god. Third, they glory in their shame. They reverse God's morality and are proud of it! Fourth, they "set their minds on earthly things" (3:19). This was their capstone. An earthly mindset ignores eternity and this effectively renders the cross irrelevant. Payment for sin (the cross) becomes irrelevent if you recognize no eternal accountability to God. A purely earthly life needs no cross.
Our citizenship is in heaven (20-21): But the reality is that the cross of Christ is relevant because there is an eternal accountability to God. And it is on the cross that Christ accomplished for us the forgiveness of sins and eternal life in heaven. And so, we live life with the hope of heaven and Christ's return. "We eagerly wait for a Savior" to come out of heaven. Paul describes this hope as "citizenship" which the Philippians understood well. For the Philippian believers, their privilege and protection were not to be found in the power of the Roman empire, but in the person of Jesus Christ. No earthly superpower could ever do anything about the reality of death and the deterioration of our bodies and the eternal punishment for sin. It is the Savior alone who can subject all things to Himself, who has the real power to transform "the body of our humble state into conformity with the body of His glory" (3:20-21). We must ever be wise and not fall into the snares of "the worries and riches and pleasures of this life" (Luke 8:14). These are no substitutes for the glories of Christ in heaven. Only a fool would settle for this world and forfeit his soul for all eternity. We keep our eyes fixed on heaven.
 These are people "who walk according to the pattern you have in us" (3:17).
 Paul's command is very direct. He literally says: "be my imitators."
 The verb "observe" (NASB) is sko-pe-ō (σκοπέω), which means "to watch," or "to keep your eyes on" (ESV).
 Verse 18 begins the first explanation ("For…") and verse 20 the second explanation ("For…").
 Paul mentioned these people before ("often told you"). This group seems to be distinct from "the dogs, the evil workers…" of 3:2, because the traits listed here seem more pagan than Jewish, but they may be included.
 This was also the end of the "opponents" of 1:28 who would face "destruction [a-pō-lei-a (ἀπώλεια)]."
 This is literally "belly [koi-li-a (κοιλία)]," an enslavement to food (1 Cor. 6:13) or other lusts (Rom. 16:18).
 "Their shame" is the sin they should have been ashamed of. They boasted and paraded their sin (Rom. 1:32).
 He emphasizes the distinction of believers from the enemies by placing the word "our" in a forward position.
 Christ will literally come out (ex hou [ἐξ οὗ]) of the heavens.
 Philippi was given the highest privilege for a Roman colony and its citizens were full Roman citizens.
 "For whoever wishes to save his life will lose it, but whoever loses his life for My sake and the gospel's will save it. For what does it profit a man to gain the whole world, and forfeit his soul?" (Mark 8:35-36).