Jeremiah 12:5 (NKJV)
Years ago, when I first started doing running sports in high school, I remember that for the first couple of days I was so tired out and my legs ached. But as I spent more and more time running it got easier and easier and I was not worn out as much. As I continued to train and do more running I got to the point where I was running races of 3 miles (5.1 kilometers). To just run that distance at first was difficult, but through practice I was eventually running races at that distance as well as running longer and longer distances.
Being a Christian, being a follower of Jesus, is a lifelong relationship, and in this relationship we should be growing and developing. The Apostle Paul wrote about the need for spiritual growth in places like Ephesians 4:14-15. In the same way that it would be bad for someone to remain a baby naturally, it would be bad for someone to remain a baby spiritually. Just like I had to train to run further distances and babies need to grow up naturally, we need to grow and develop spiritually so we can run the race that God has called us to (Hebrews 12:1).
Wherever we are now spiritually, we need to grow to do more in the future; we cannot stay at the level we are at now and expect to do more for God. If we find ourselves in a struggle just to live at the spiritual level we are at now, how will we ever do more in the future? If we are not growing spiritually and we are not developing in our faith, the time will come when God will ask us to do something and we will not be ready to do it. We need growth and development as believers so that we are equipped to do the things that God has called us to do.