Lord Shiva is the best knower of Brahman.
This sloka is from the discourse given by Shree Bhishma to Yudhishithira
Pramaan from Bhagavatam.....
asyänubhävam bhagavän veda guhyatamam sivah
devarsir näradah säksäd bhagavän kapilo nrpa
O King, Lord Siva, Närada the sage amongst the demigods, and Kapila, theincarnation of Godhead, all know very confidentially about His glories throughdirect contact.
Narada Purana (63.121) says:
diksaya guru-murti-sthah sarvanugrahakah sivah
Through diksa, Shiva, the giver of all benedictions, is manifested in the form of the guru.
om namas te natha bhagavan sivaya guru-rupine Narada Purana (65.58):
I offer my respectful obeisances unto Lord Shiva, who has assumed the form of guru.
I hope it is clear to you.
Pranaam