That could possibly work in some places, but it the goal is to hide all of the content on the page and replace it with a login form, content form ACF fields can be shown in many ways and that would only cover the content of one field. Most of the custom fields are shown, as you know, by adding additional HTML and templates that is not effected by the_content or acf_the_content. I think that the only way to ensure that nothing is output on the page is to add a conditional around all of the HTML and acf function calls so that the code is not run if the user must log in or is for some other reason not permitted to see the content and to only run the_content() in the template.
