Load and activate of Emacs package.el - emacs

Question

I have something like (to be simplify)
(require 'package)
(setq package-archives
'(("org" . "http://orgmode.org/elpa/")
("melpa" . "http://melpa.org/packages/")
("melpa-stable" . "http://stable.melpa.org/packages/")
("gnu" . "http://elpa.gnu.org/packages/")))
(setq package-enable-at-startup nil)
(package-initialize t)
(unless (package-installed-p 'use-package)
(package-refresh-contents)
(package-install 'use-package))
(let ((default-directory (expand-file-name user-emacs-directory "elpa")))
(normal-top-level-add-subdirs-to-load-path))
(require 'use-package)
(use-package helm
:ensure t
:commands (helm-M-x
helm-mini
helm-find-files
helm-show-kill-ring)
:init
(add-hook 'after-init-hook 'helm-mode)
:config
(helm-autoresize-mode 1))
in my .emacs. But I was told that Symbol's function definition is void: helm-mode after startup emacs.
After reading the manual of Packaging Basics and this answer, I find I still don't understand the mechanism of package.el.
I notice that the words load and activate are used in the manual, I suppose that load is 'Emacs adds the package’s content directory to load-path, and evaluates the autoload definitions in name-autoloads.el.' and activate is to 'fully require'. I know if I change (package-initialize t) into (package-initialize) everything will be fine.
I confirm that something like "/Users/gaki/.emacs.d/elpa/helm-20160112.258" is in load-path and helm-mode is autoload. Isn't it can be autoload and even in the after-init-hook ?

Answer 1

A look at the code for package-activate and its helper package-activate-1 shows that a package's autoloads file is not loaded unless the package has been 'activated'.
You've told Emacs not to activate any packages by using (package-initialize t), and you've prevented it from activating them after loading your init file with (setq package-enable-at-startup nil), so none of your package autoloads are known to Emacs.
I suggest changing (package-initialize t) to (package-initialize).
Note that the documentation you linked to describes the NO-ACTIVATE argument as "for internal use only".
p.s. Activation does not require the package features -- there would be no purpose to the autoloads if it did -- so this should not take a very significant amount of time, unless perhaps you have a really large number of packages? What kind of time difference are you actually seeing here? (https://emacs.stackexchange.com/a/19263/454 may help with answering that.)