When should I use `require` a package which is `autoload`?

Some blogs/threads will use

(require 'evil)
(evil-mode 1)

to enable evil mode.

AFAIK, Elisp is lisp2 and evil-mode is also a function, so doesn't need setq.

(defadvice evil-mode (after start-evil activate)
  "Enable Evil in Fundamental mode."
  (if evil-mode
      (progn
        (when (eq (default-value 'major-mode) 'fundamental-mode)
          ;; changed back by `evil-local-mode'
          (setq-default major-mode 'turn-on-evil-mode))
        (ad-enable-regexp "^evil")
        (ad-activate-regexp "^evil")
        (with-no-warnings (evil-esc-mode 1)))
    (when (eq (default-value 'major-mode) 'turn-on-evil-mode)
      (setq-default major-mode 'fundamental-mode))
    (ad-disable-regexp "^evil")
    (ad-update-regexp "^evil")
    (with-no-warnings (evil-esc-mode -1))))

But， because of autoload,

(evil-mode 1)

Without require, it also works fine(at least for me)

(If anywhere below is wrong, please correct me, thx)

My question is when must I use require if the required package is autoloaded

Solution

You're correct -- if an autoload declaration for evil-mode has been evaluated, then there is no need to (require 'evil) prior to (evil-mode 1)

Individual users may or may not need to (require 'evil) though. It's a question of how the library has been installed. On its own an ;;;###autoload cookie does nothing; but if you installed evil using package.el, then they will have been parsed and turned into a file of autoload declarations, which Emacs will load when packages are initialised.

If you install a library without using a package manager, then you may need to require it, or else write your own autoload declaration for the functions you might be calling.