You will develop spiritual muscles as you resist the enemy’s temptations.
Read: I Corinthians 15:58
“So, my dear brothers and sisters, be strong and immovable. Always work enthusiastically for the Lord, for you know that nothing you do for the Lord is ever useless.”
http://bibleresources.bible.com/passagesearchresults2.php?tp=16&book_id=53&c=15&passage1=1+Corinthians+15%3A58&version1=51