You can keep full post, page or a few lines visible only to your registered site users whereas display a Login to Readmore link for other site visitors.
So first install and activate the new WordPress plugin Login to read more. After activation you can simply enclose your text inside
[auth] & [/auth] as shown in the following example:
[auth]This content will display for registered users only[/auth]
How it works ?
Text enclosed in shortcode tags shows up a login to readmore button to non-logged in users of your website. The button link directs them to wp-login page and if they are registered site users they can simply login and the plugin will redirect them to the content they were trying to read.
Arguments & Filters: The plugin provides you few parameters and filter hooks, visit its WordPress plugin page for more information.