But those are preaching Christ in a spirit of competition, not from their hearts, but with the purpose of giving me pain in my prison.

Because to you it has been given in the cause of Christ not only to have faith in him, but to undergo pain on his account:

You have paid me in full, and more too. I am fully supplied with what I have received from you through Epaphroditus. It is like fragrant incense, just such a sacrifice as God welcomes and approves.