Error When Previewing/Activating New Theme

I am trying to activate a new theme on my website, and keep getting following error message: Fatal error: Cannot redeclare the_content_limit() (previously declared in /home2/lakeside/public_html/wordpress/wp-content/plugins/limit-post.php:18) in /home2/lakeside/public_html/wordpress/wp-content/themes/church_30/functions.php on line 360

How would I resolve this problem? thanks so much!