Our spiritual walk is compared in several passages to the training and performance of an athlete.
I may never have been on a competitive sports team, but I understand the discipline; for in my teens, I used to distance run just for the pleasure of it.
Aside from setting a maintainable pace and good breathing, one of the most important things I learned is that the runner must keep his or her eyes forward, or risk tripping and falling.
I'd learned that lesson as a preschooler, the first (and last) time I ever ran from Granny. I was in trouble and trying to get away from her; while looking back to see if she was gaining on me, I tripped over a tree stump. She was on me with that willow tree switch in an instant, "like white on rice!"
We will not grow spiritually if we are constantly looking backwards.
In a similar comparison, Jesus said that a man who sets his hand to the plow and looks back is not fit for service in the kingdom of God (Luke 9:62). Why?
Because a farmer who doesn't keep his eyes forward can't possibly plow a straight furrow; a runner can't possibly aim for the finish line if he's glancing over his shoulder, and a Christian who constantly looks back will be all over the place, instead of...