Devise\Users\UserManager::activate PHP Method

    public function activate($user)
    {
        $user->activated = true;
        $user->activate_code = null;
        return $user->save();
    }

 /**
  * Process user activation request.
  *
  * @param  integer  $userId
  * @param  string   $activateCode
  * @return False | DeviseUser
  */
 public function activate($userId, $activateCode)
 {
     $user = $this->UsersRepository->findById($userId);
     if ($activateCode === $user->activate_code) {
         $this->UserManager->activate($user);
         // activate the user
         $this->Auth->login($user);
         // auto-login newly activated user
         $this->message = 'Account successfully activated.';
         return true;
     }
     $this->message = 'Issues occurred while attempting to activate account. Please contact support.';
     return false;
 }