We all have times and seasons of great spiritual reward, joy and power. None of us have them continually. They ebb and flow, so the quotes here are to prepare us for when the Spirit seems to have left forever.  

 "He who is too secure in time of peace will often be found too much dejected and fearful in time of war. It is good counsel that when you have received the spirit of fervor that you should meditate how it will be with you when that light shall leave you. And when it shall happen, remember that the light may return again, which, for your instruction and God's glory, He withdrew from you for a time." Thomas A Kempis

 "The wind blows where it wishes, and you hear the sound of it, but cannot tell where it comes from and where it goes. So is everyone who is born of the Spirit.” Jn. 3:8