The author teaches here a very important lesson. Dhikr is the heart of the Sufi order. Allah ordered the servants to remember Him as often as possible.
Dhikr can be recited either Jaliy (loud) or Khafiy (silently in the heart). Both are rewardable. The goal is to purify the heart or soul. Through Dhikr one gains the object of life, happiness and satisfaction of Allah.
Imam Nawawi says: Dhikr is with the heart (Khafi) and with the tongue (Jaliy), and the best type of Dhikr is to do it with the heart and the tongue together. If he limits it to one of them, then the (Dhikr with) heart is better.
The author also points to Mujahada, which means striving hard and training one’s soul to obey Allah. He needs to do a lot of tough exercises to get through Mujahada so that he enters the status of ‘Mushahada’.