A Spiritual Master enters the life of a person at an opportune moment. It is pre-destined. It could happen in the case of the person who has performed enormous meritorious deeds and as such has accumulated merit (punya). It could also happen in the case of a sinner who has committed many sins and needs to be reformed.
In the former case, the Sadguru liberates the person from the shackles called worldly bondages and makes him realize his higher Self. In the latter case, the Guru first starts as a healer. Firstly, he ensures that there is no further moral deterioration. He then brings a subtle but sure change in the person concerned. The process may be slow and not explicitly visible in the beginning.
The Guru may or may not take a physical form. Shakti or spirit may manifest in the form of sitting in the conscience or in the form of a revelation or experience as happened to Gautam Buddha.
God is the creator of everything in this universe, including the Guru. He is of infinite compassion. However, such Lord leaves us to act as we like and reap the fruits of our actions, good or bad.
Spiritual Guru on the other hand, is our friend, philosopher and guide. He shows the path that leads to God and guides us step by step on that path. He is therefore the great God himself. Guru is Brahma, Vishnu and Shiva himself!
(Bhaktimala Jan 1977)