If You have added Modular Front Page contribution, TEXT_MAIN is in 'includes/modules/frontpage/text_main.php'
Simply add to function execute():
$body_text .= ' <div class="contentText">' . PHP_EOL;
$body_text .= $pagetext . PHP_EOL;
$body_text .= ' </div>' . PHP_EOL;
and change this line in this function:
global $oscTemplate, $language, $PHP_SELF, $cPath;
to:
global $oscTemplate, $language, $PHP_SELF, $cPath, $pagetext;
(Added ', $pagetext' at the end of line)