Just a short verse but take a good look at what Paul is saying. He is making it clear that these believers had learned enough about Christ to know that living like they used to simply wasn't an option if one was going to please the Lord. He is also implying that these believers had taken time to increase in their personal knowledge of Jesus. And this was far from academic knowledge. This was the kind of knowledge you get from taking the time to walk and talk with that person. This is a practice that far too few Christians engage in as they should. To really know Christ takes time, time alone with Him, time to meditate on Him as you would after spending time with the person you're in love with. If you had a great date with someone, you dwell on the experience long after the date was over. You're still thinking about it when you're retiring for the night or you're heading off to work the next morning.
Is that how you think about your last time of prayer with Jesus? When you closed your Bible and headed off to work did you think and rethink about what you just read? Or when you put your head on your pillow do you fall asleep with a joy in your heart because of what you've experienced with your best Friend?! May God give you precious times with Him as you grow in your Christian life!