It’s the same with Christians. We mature when we let God use us for the things He designed us for. That’s often difficult; it puts us between God and tough situations, feeling pressure from both sides. But that’s what takes us from stiff to supple. Just like jeans, we can’t get there if we’re sitting on the shelf.
The same thing happens to maturing Christians; we must regularly get into God’s Word and conversation with Him. That not only cleans us for our next use, but each session takes out a little more of our original, sinfull, nature, making it less and less prominent.