From Chapter IV of Bhakti-Yoga of Volume 3 of Complete Works of Swami Vivekananda

Every soul is destined to be perfect, and every being, in the end, will attain the state of perfection. Whatever we are now is the result of our acts and thoughts in the past; and whatever we shall be in the future will be the result of what we think end do now. But this, the shaping of our own destinies, does not preclude our receiving help from outside; nay, in the vast majority of cases such help is absolutely necessary. When it comes, the higher powers and possibilities of the soul are quickened, spiritual life is awakened, growth is animated, and man becomes holy and perfect in the end.

This quickening impulse cannot be derived from books. The soul can only receive impulses 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 that we have not developed at all spiritually. It is not true that a high order of intellectual development always goes hand in hand with a proportionate development of the spiritual side in Man. In studying books we are sometimes deluded into thinking that thereby we are being spiritually helped; but if we analyse the effect of the study of books on ourselves, we shall find that at the utmost it is only our intellect that derives profit from such studies, and not our inner spirit. This inadequacy of books to quicken spiritual growth is the reason why, although almost every one of us can speak most wonderfully on spiritual matters, when it comes to action and the living of a truly spiritual life, we find ourselves so awfully deficient. To quicken the spirit, the impulse must come from another soul.

The person from whose soul such impulse comes is called the Guru — the teacher; and the person to whose soul the impulse is conveyed is called the Shishya — the student. To convey such an impulse to any soul, in the first place, the soul from which it proceeds must possess the power of transmitting it, as it were, to another; and in the second place, the soul 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 genuine religion takes place. “The true preacher of religion has to be of wonderful capabilities, and clever shall his hearer be” —

आश्चर्यो वक्ता कुशलोऽस्य लब्धा ;

and when both of these are really wonderful and extraordinary, then will a splendid spiritual awakening result, and not otherwise. Such alone are the real teachers, and such alone are also the real students, the real aspirants. All others are only playing with spirituality. They have just a little curiosity awakened, just a little intellectual aspiration kindled in them, but are merely standing on the outward fringe of the horizon of religion. There is no doubt some value even in that, as it may in course of time result in the awakening of a real thirst for religion; and it is a mysterious law of nature that as soon as the field is ready, the seed must and does come; as soon as the soul earnestly desires to have religion, the transmitter of the religious force must and does appear to help that soul. When the power that attracts the light of religion in the receiving soul is full and strong, the power which answers to that attraction and sends in light does come as a matter of course.

There are, however, certain great dangers in the way. There is, for instance, the danger to the receiving soul of its mistaking momentary emotions for real religious yearning. We may study that in ourselves. Many a time in our lives, somebody dies whom we loved; we receive a blow; we feel that the world is slipping between our fingers, that we want something surer and higher, and that we must become religious. In a few days that wave of feeling has passed away, and we are left stranded just where we were before. We are all of us often mistaking such impulses for real thirst after religion; but as long as these momentary emotions are thus mistaken, that continuous, real craving of the soul for religion will not come, and we shall not find the true transmitter of spirituality into our nature. So whenever we are tempted to complain of our search after the truth that we desire so much, proving vain, instead of so complaining, our first duty ought to be to look into our own souls and find whether the craving in the heart is real. Then in the vast majority of cases it would be discovered that we were not fit for receiving the truth, that there was no real thirst for spirituality.

There are still greater dangers in regard to the transmitter, the Guru. There are many who, though immersed in ignorance, yet, in the pride of their hearts, fancy they know everything, and not only do not stop there, but offer to take others on their shoulders; and thus the blind leading the blind, both fall into the ditch.

अविद्यायामन्तरे वर्तमानाः स्वयं धीराः पण्डितम्मन्यमानाः । दन्द्रम्यमाणाः परियन्ति मूढा अन्धेनैव नीयमाना यथान्धाः ॥ — “Fools dwelling in darkness, wise in their own conceit, and puffed up with vain knowledge, go round and round staggering to and fro, like blind men led by the blind.” — (Katha Up., I. ii. 5). The world is full of these. Every one wants to be a teacher, every beggar wants to make a gift of a million dollars! Just as these beggars are ridiculous, so are these teachers.