ঈশ্বরো গুরুরাত্মেতি মূর্তিভেদবিভাগিনে

This is a verse from Manasollasa (Section 1, Verse 30), an extensive commentary on Sri Dakshinamurthy Stotram, written by Sri Sureshvaracharya, a disciple of Sri Adi Sankaracharya (who composed Sri Dakshinamurti Stotram). Sri Dakshinamurthy is not only worshiped as the first Guru, but also as the One, who teaches everyone through the medium of innumerable number of human Gurus. The Lord himself is in this three-fold form as Iśvara, Guru and atman, the Self. Mürti-bheda-vibhāgine, the one who remains 'as though' divided in this three-fold form. Like what? Vyomavad vyāpta-dehāya Vyomavat, like space.