Restrict specific posts in your WordPress site for logged in users only.

my_restricted_posts_require_user_login.php

<?php
/**
 * Restrict specific posts in your WordPress site for logged in users only.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
function my_restricted_posts_require_user_login() {
	global $current_user;

	// Allow logged in users.
	if ( is_user_logged_in() ) {
		return;
	}

	// Restrict non-users from accessing posts with these IDs:
	$restricted_posts = array( 1, 2, 3 );
	
	if ( is_single( $restricted_posts ) ) {
		wp_redirect( home_url( 'wp-login.php?redirect_to=' . urlencode( $_SERVER['REQUEST_URI'] ) ) );
	} else {
		return;
	}
}
add_action( 'template_redirect', 'my_restricted_posts_require_user_login' );