first paragraph of the_content as meta description

You can use the_excerpt() instead of the_content

Reference link for filter your excerpt : show-first-paragraph
Or you can also do this by callback function inside your loop using the function to modify the_content as below:

function get_first_paragraph(){
    global $post;

    $str = wpautop( get_the_content() );
    $str = substr( $str, 0, strpos( $str, '</p>' ) + 4 );
    $str = strip_tags($str, '<a><strong><em>');
    return $str;
}

Hope this helps!!