by Swami Vivekananda

How, then, are we to know a teacher? The sun requires no torch to make it visible; we need not light a candle in order to see it. When the sun rises, we instinctively become aware of the fact, and when a teacher of men comes to help us, the soul will instinctively know that truth has already begun to shine upon it.

Truth stands on its own evidence; it does not require any other testimony to demonstrate it. It is self-effulgent. It penetrates into the innermost corners of our nature, and in its presence the whole universe stands up and says, "This is truth."

Those teachers whose wisdom and truth shine like the light of the sun are the very greatest the world has known, and they are worshipped as God by the major portion of mankind. But we may get help from comparatively lesser teachers also; only we ourselves do not possess intuition enough to judge properly of the man from whom we receive teaching and guidance. So there ought to be certain conditions, for the teachers to satisfy, as also for the taught (disciple).

The conditions necessary for the taught are purity, a real thirst after knowledge, and perseverance. No impure soul can be really religious. Purity  in thought, speech, and act is absolutely necessary for anyone to be religious.  As to the thirst for knowledge, it is an old law that we all get only what we want. None of us can get anything other than what we fix our hearts upon. To pant for religion is truly a very difficult thing; it is not as easy as we generally imagine. Hearing religious talks, reading religious books, is no proof yet of a real want felt in the heart. There must be a continuous struggle, a constant fight and unremitting grappling with our lower nature, till the higher is actually felt and the victory is achieved (Abhayas Yoga). It is not a question of one or two days, of years, or of lives; the struggle may have to go on for hundreds of lifetimes. Success may sometimes come immediately, but we must be ready to wait patiently even for what may look like an infinite length of time. The student who sets out with such a spirit of perseverance will surely find success and realization at last (Abhyas Yoga).

With regard to the teacher, we must see that he knows the spirit of the scriptures. The whole world reads Bibles, Vedas, and Korans; but they are all only words, syntax, etymology, philology -- the dry bones of religion. The teacher who deals too much in words and allows the mind to be carried away by the force of words loses the spirit.

It is knowledge of the spirit of the scriptures, alone, that characterizes the true religious teacher. The network of words of the scriptures is like huge forest in which the human mind often loses itself and finds no way out.

"The network of words is a big forest; it is the cause of aimless wandering of the mind. The various methods of joining words, the various methods of speaking in beautiful language, the various methods of explaining the diction of the scriptures are only for the disputations and enjoyment of the learned; they do not conduce to the development of spiritual perception."

Those who employ such methods to impart such religion to others are only desirous to show off their learning, so that the world may praise them as great scholars (or gurus).

You will find not one of the great teachers of the world ever went into these various explanations of the texts.