2:5-11 - "Let the same mind be in you that was in Christ Jesus, 'who, though he was in the form of God, did not regard equality with God as something to be exploited, but emptied himself, taking the form of a slave, being born in human likeness. And being found in human form, he humbled himself and became obedient to the point of death--even death on a cross. Therefore God also highly exalted him and gave him the name that is above every name, so that at the name of Jesus every knee should bend, in heaven and on earth and under the earth, and every tongue should confess that Jesus Christ is Lord, to the glory of the Father'" (NRSV).
--This passage comes in the context of an exhortation to "in humility consider regard others as better than yourselves" (2:3). Thus the emphasis here is on Christ's humility in his incarnation. But his obedience is also a factor, obedience "to the point of death--even death on a cross." It is Jesus' obedience that causes the Father to exalt him. Thus the cross is the supreme act of obedience to the Father, one that reflects and culminates a lifetime of perfect devotion.
3:10-11 - "I want to know Christ and the power of his resurrection and the sharing of his sufferings by becoming like him in his death, if somehow I may attain the resurrection from the dead."
--Cf. Colossians 1:24. Paul desires to share in Christ's sufferings, which may point to a sacramental "participation in the divine nature" (2 Peter 1:4), in which atonement is effected by our sharing in the life of Jesus, which itself was offered as a substitution for ours. In any case, obviously there is some aspect of Christ's suffering and death in which Paul expects Christians to be able to participate.
3:18 - "For many live as enemies of the cross of Christ; I have often told you of them, and now I tell you even with tears. Their end is destruction; their god is the belly; and their glory is in their shame; their minds are set on earthly things."
--Paul here defines what it means to be an enemy of the cross. Can we draw conclusions from this passage on what it then means to be a "friend" of the cross (to have "citizenship in heaven," 3:20)?