If a visitor is not logged into the WordPress site, send them directly to the login page. After they have logged in, redirect them back to the home page.

Redirect on Login.php

// Place in functions.php or in a plugin

function redirection() {
  if ( ! is_user_logged_in() ) {
	  wp_redirect( '/wp-login.php?redirect_to=' . home_url() );
	  exit;
  }
} // end function

add_action( 'get_header', 'redirection' );