Add a custom text message to WordPress login page. Source: https://codex.wordpress.org/Plugin_API/Filter_Reference/login_message

login-message.css

.login-message {
    color: #777;
    font-size: 14px;
    line-height: 1.625;
    padding: 12px 24px 24px;
}

login-message.php

<?php

//* Add a custom text message to WordPress login page
function prefix_login_message($message) {
    if (empty($message)) {
        return "<p class=\"login-message\">Add your custom text message here!</p>";
    } 
    else {
        return $message;
    }
}
add_filter('login_message', 'prefix_login_message');

//* Load "login-message.css" CSS file stored in active child theme directory
function prefix_login_stylesheet() {
    wp_enqueue_style('custom-login', get_bloginfo('stylesheet_directory') . '/login-message.css');
}
add_action('login_enqueue_scripts', 'prefix_login_stylesheet');