Paul describes them in various ways.
They are immature Christians.
They use gospel to promote themselves (1:16-17),
They lack humility, kindness, and compassion for others (2:1-4),
They are selfish (2:3),
Their attitude towards other is evil (2:5),
They cause division and contention in the Church (4:2).
They are saved but they are not working out their salvation with fear and trembling. They have taken the present salvation for granted (2:12). Their confidence is in themselves, not in Christ. The have forgotten the life of Jesus Christ as their model to emulate.
Others are carnal Christians. Concerning them Paul writes in Phil 3:18-19,
For, as I have often told you before and now say again even with tears, many live as enemies of the cross of Christ. 19 Their destiny is destruction, their god is their stomach, and their glory is in their shame. Their mind is on earthly things.
Paul is saying, “Watch out! You may have come very close to those
who live like the enemies of the cross of Christ
whose destiny is destruction
whose god is their stomach
whose glory is shame
whose mind is on earthly things
Still others who promotes the practice Jewish circumcision for salvation
Regarding them Paul writes in Phil 3:2,
2 Watch out for those dogs, those men who do evil, those mutilators of the flesh.
Whatever the case may be, it is obvious that the Christians at Phillipi have put aside the life of Jesus aside and so there is no Christ likeness.
They have division among themselves.
This is the context in which Paul writes about his loss and gain philosophy to the church at Philipi.
He wants the church at Philipi to understand is that knowing Jesus Christ is a business of loss not gain. And it must be continual loss compared to surpassing greatness of knowing Christ.