Can be done easily with a shortcode:
[members-only]...content...[/members-only]
The shortcode will return content for members, or text ‘<h3>This is for members only!</h3>‘, for non member request. The callback function of the shortcode will check who made a request, and return display data accordingly. Any other text on page/post will be displayed as is. For more info about shortcodes see: Shotcodes API