Paul is continuing on here on his theme from verse 8:
Christians are told that we have been (past tense) filled up with Christ. Imagine taking your car that was just filled up with the best quality gasoline, and pouring out a good deal of that gasoline on the ground and replacing it with water. That sounds crazy, but it's a good image of what we do when we replace some of the Spirit in us with something lesser from this world. Just like the car filled with water, we won't operate any better by being filled up with the empty philosophies of this world. Actually, we've made a rotten trade that will only destroy us in the end. It will hold us captive, and we will be more useless than a car with a tank full of water (or sawdust, or grape juice, or any other substance that isn't what it was made to be filled up with)
We were designed to be filled up with Christ, and He has freely given us that 'filling up', one that is more precious than any price per gallon of our natural gasoline. Are we using it? Or are we trying to replace His blessings with the empty deceits of this world?