It is interesting to me how our bodies handle stress. I used to weight train and my muscles responded positively to good stress. (There are some that might read this and ask, "Is there such a thing as good stress"?) Those same muscles respond very differently when I am fatigued and trying to function on little rest and a lot of caffeine. Those same muscles will atrophy (shrink, if you will) if not used at all.
Our faith is sort of like this. When we are challenged to strengthen our faith and have a steady routine of prayer, Bible study and fellowship so our faith matures and we grow stronger. But that same faith, if not properly exercised will atrophy. You will not loose the faith, so to speak, but you will loose the impact and strength you, and others, once benefited from.
If you don’t spend enough time in the spiritual gym, get in there. You will not only benefit from it but so will those you minister to.