Paul contrasts those who 'mind earthly things' with a radical alternative. What does it mean to have your 'citizenship' somewhere else?
Paul uses a powerful Greek word, politeuma, to describe the Christian's identity. It doesn't just mean where you live, but the entire framework of your society: its laws, its government, its way of life.
For the Philippian believers, and for us today, this means our primary allegiance, our ultimate belonging, isn't to any earthly nation or system. Our 'commonwealth' is in heaven.
This doesn't mean we ignore earthly responsibilities, but that our perspective, our values, and our ultimate hope are rooted in God's heavenly kingdom. We are 'in the world' but not 'of the world.' We live by heavenly principles, even while walking on earthly ground.