How to restrict a page [without plugin]

You can do this pretty easily with a shortcode. Hook into init and add the shortcode in your hooked function.

<?php
add_action('init', 'wpse57819_add_shortcode');
/**
 * Adds the shortcode
 *
 * @uses add_shortcode
 * @return null
 */
function wpse57819_add_shortcode()
{
    add_shortcode('restricted', 'wpse57819_shortcode_cb');
}

Then in your callback function, you can check to see if the user is logged in. If they are, show them the content. If not, show them a login message. You can do literally whatever you want here: check for user capabilities to show them the content (different “membership levels”), show them an entire login form. A simple example:

<?php
/**
 * Callback function for the shortcode.  Checks if a user is logged in.  If they
 * are, display the content.  If not, show them a link to the login form.
 *
 * @return string
 */
function wpse57819_shortcode_cb($args, $content=null)
{
    // if the user is logged in just show them the content.  You could check
    // rolls and capabilities here if you wanted as well
    if(is_user_logged_in())
        return $content;

    // If we're here, they aren't logged in, show them a message
    $defaults = array(
        // message show to non-logged in users
        'msg'    => __('You must login to see this content.', 'wpse57819'),
        // Login page link
        'link'   => site_url('wp-login.php'),
        // login link anchor text
        'anchor' => __('Login.', 'wpse57819')
    );
    $args = wp_parse_args($args, $defaults);

    $msg = sprintf(
        '<aside class="login-warning">%s <a href="https://wordpress.stackexchange.com/questions/57819/%s">%s</a></aside>',
        esc_html($args['msg']),
        esc_url($args['link']),
        esc_html($args['anchor'])
    );

    return $msg;
}

As a plugin.

Usage

Somewhere in your pages/posts:

[restricted]
Content for members only goes here
[/restricted]