Every soul is destined to be perfect, and every being, in the end, will attain that state. Whatever we are now is the result of whatever we have been or thought in the past; and whatever we shall be in the future will be the result of what we do or think now. But this does not preclude our receiving help from outside; the possibilities of the soul are always quickened by some help from outside, so much so that in the vast majority of cases in the world, help from outside is almost absolutely necessary. Quickening influence comes from outside, and that works upon our own potentialities; and then the growth begins, spiritual life comes, and man becomes holy and perfect in the end. This quickening impulse which comes from outside cannot be received from books; the soul can receive impulse only from another soul, and from nothing else. We may study books all our lives, we may become very intellectual, but in the end we find we have not developed at all spiritually…
Now, in intellectual development, we can get much help from books, but in spiritual development, almost nothing… That is the reason why almost every one of us can speak most wonderfully on spiritual subjects, but when the time of action comes, we find ourselves so woefully deficient. It is because books cannot give us that impulse from outside. To quicken the spirit, that impulse must come from another soul. That soul from which this impulse comes is called the Guru, the teacher; and the soul to which the impulse is conveyed is called the disciple, the student. In order to convey this impulse, in the first place, the soul from which it comes must possess the power of transmitting it, as it were, to another; and in the second place, the object to which it is transmitted must be fit to receive it. The seed must be a living seed, and the field must be ready, ploughed; and when both these conditions are fulfilled, a wonderful growth of religion takes place. "The speaker of religion must be wonderful, so must the hearer be"; and when both of these are really wonderful, extraordinary, then alone will splendid spiritual growth come, and not otherwise. These are the real teachers, and these are the real students. Besides these, the others are playing with spirituality — just having a little intellectual struggle, just satisfying a little curiosity — but are standing only on the outward fringe of the horizon of religion.
… There is nothing higher and holier than the knowledge which comes to the soul transmitted by a spiritual teacher...These teachers of all teachers represent God Himself in the form of man. They are much higher; they can transmit spirituality with a touch, with a wish, which makes even the lowest and most degraded characters, saints, in one second. Do you not read of how they used to do these things? ...they are the Teachers of all teachers, the greatest manifestations of God to man; we cannot see God except through them.
(Complete Works of Swami Vivekananda, Vol. 4)