Source: Talks with Sri Ramana Maharshi
God takes the form of a Guru and appears to the devotee; teaches him the Truth; purifies the mind by his teachings and contact
Devotee: What is Guru’s Grace? How does it work?
Bhagawan Ramana Maharishi.: Guru is the Self.
Devotee: How does it lead to realisation?
Bhagawan Ramana Maharishi.: Isvaro gururatmeti … (God is the same as Guru and Self …). A
person begins with dissatisfaction. Not content with the world he seeks satisfaction of desires by prayers to God; his mind is purified; he longs to know God more than to satisfy his carnal desires. Then God’s Grace begins to manifest. God takes the form of a Guru and appears to the devotee; teaches him the Truth; purifies the mind by his teachings and contact; the mind gains strength, is able to turn inward; with meditation it is purified yet further, and eventually remains still without the least ripple. That stillness is the Self.
The Guru is both exterior and interior. From the exterior he gives a push to the mind to turn inward; from the interior he pulls the mind towards the Self and helps the mind to achieve quietness. That is Grace. Hence there is no difference between God, Guru and Self.