How do I change the display of a class using JavaScript?

A suggestion, why not use JQuery. It's quite simple and easy, style can be none/inline/block etc

$('.className').css('display','style');

Show:

document.querySelectorAll(".login").forEach(a=>a.style.display = "initial");

Hide:

document.querySelectorAll(".login").forEach(a=>a.style.display = "none");

Changing display of every element is bad idea. Just toggle class of all these logins wrapper and set display in CSS.

A bit late now but this gets some views. Why not:

var arrayOfElements=document.getElementsByClassName('login');
var lengthOfArray=arrayOfElements.length;

for (var i=0; i<lengthOfArray;i++){
    arrayOfElements[i].style.display='inline';
}