Is a teacher necessary for Spiritual Realization?
There are no rules. The teacher in the form of a person is only one of many possible.
The teacher in the form of a person obviously can provide consistent intellectual articulation and emotional resonance. Human interaction is a very special thing indeed because of empathy. The teacher has lived the suffering the seeker is living through. And, in the case of spiritual realization, understands in direct experience how the end of suffering comes about.
What other forms are possible?
It depends on our degree of de-identification with the ego, degree of consciousness of where we are on the path and our ability to read the messages. Tarot, telepathic connection with guides, channeling or mediunic communication with them, books, videos, audio; these are all possible teachers.
The Truth can be Glimpsed through any stimulus. In my case it was after reading a paragraph in a book. Only later did I go after teachers to ask for an explanation of certain aspects of the process I was going through.
If the Glimpse of Truth has already happened one knows “where one is going” so the pointers will come to lead one in that direction of finalizing the process of dissolution of falseness. If the Glimpse has not yet happened then the pointers will show us what must be seen in order to strengthen true personality and prepare for the Glimpse.
Our husband or wife can be the teacher. A friend, the beggar in the street, the cashier at the supermarket, our dog or cat, the sky, a flower, the oil spill on the pavement, anything. Everything teaches. The less self-centered fear there is the more powerful teachers become available.
The fact of being conscious itself is a teacher. In a state of enough sensitivity everything is teaching us something at all times.
I read somewhere that the sage Milarepa gave an interesting answer to that question. He purportedly said: “I have twenty eight teachers including the four elements.” That means he was in a state of un-involvement with the ego such that any and everything in his life was pointing the way, and he could hear it.