Jess

A simple plugin that allows you to restrict access to certain pages on your website to only logged in users quickly and easily. This plugin can be useful to those who do not need overly complicated membership systems, and to those who want to password protect a group of pages but don’t want to password protect each page individually, forcing the users to log into each one.
It is simple to use. First select whether you want to password protect all pages by default, and want to choose which pages are public. Or choose to allow all pages to be public by default, and choose which pages are password protected.
When non-logged in users try to access a password protected page they will be redirected to the WordPress login screen. Once logged in, they will be redirected back to the requested page and will not be asked to log in again on subsequent pages.

Password Protect Pages

Restrict access to specific pages to only logged in users.

marcucci

This is a very simple plugin that will check to see if a visitor is logged in before allowed your WordPress site to be displayed. 
If they are not it will display the login page of your site. It will also add text on the login page to explain that the site 
required all visitors to login.

Password Protect Plugin for WordPress

Force all visitors to login before accessing your wordpress site and add a message on the login screen explaining that login is required.

Password Protect Pages vs Password Protect Plugin for WordPress