From the Gospel Coalition blog:
It’s important to note that in these verses Paul doesn’t pray for something the Colossian Christians don’t have. Rather, he prays they will grow in their awareness and understanding of what they do have. Christian growth doesn’t happen by working hard to get something you don’t have. Christian growth happens by working hard to live in the reality of what you do have.
I used to think that when the Bible tells us to work out our salvation, it meant go out and get what you don’t have—get more patience, get more strength, get more joy, get more love, and so on. But after reading the Bible more carefully I now understand that real gospel fruit happens, not as we “work harder” but only as we continually rediscover the gospel. You could put it this way: rediscovering the gospel is the hard work we’re called to.
You see, the secret of the gospel is that we become more spiritually mature when we focus less on what we need to do for God and focus more on all that God has already done for us. The irony of the gospel is that we actually perform better as we grow in our understanding that our relationship with God is based on Christ’s performance for us, not our performance for him.