Redirect a to a specific page depending on whether the user is authenticated or not.

functions.php

<?php

/**
 * Callback for WordPress 'template_redirect' action.
 * 
 * http://codex.wordpress.org/Plugin_API/Action_Reference/template_redirect
 * 
 * @return none
 */
function my_custom_redirect(){
        
        // if not on specific page, continue to page (use is_single() to detect a specific post)
        if( ! is_page( 10 ) )
                return;
        
        // if the user is logged in, continue to page
        if( is_user_logged_in() )
                return;
        
        // redirect to page id 5
        wp_redirect( get_permalink( 5 ) );
        
        // (optional) prevent other action callbacks from performing addtional redirects
        exit();
        
}
add_action( 'template_redirect', 'my_custom_redirect' );