Initiation is the formal admission into an organization or society; or the ceremonies or rites of admission. A rite of passage.
     Spiritual initiation has often been known as gnosis or enlightenment. In other words, is the ultimate outcome or at least a major step on the path of those seeking spiritual enlightenment.
       In the ancient or Medieval world  there were groups of people who were looking for a spiritual experience that they called "gnosis," a Greek word that meant a kind of knowledge only gained through a spiritual initiation. The people who would have used this word to describe their spiritual goal included groups of Jews (such as the Essenes), groups of Pagans (such as the people who wrote the Hermitic texts), and groups of Christians who later came to be considered heretics by the orthodox Christians. Originally, historians used the term to refer to all of these groups.   In Christian mysticism we find four initiations or baptisms.  Earth, (birth into the material world), Water (puberty), Air (the Gnosis) and Fire, (the ordeal).  They may not occur in this order

All spiritual practice, of whatever variety, consists of elevating the kundalini; and every genuine spiritual experience--the visitation of the Lord or angels, flashes of light hearing the subtle sounds of divine music, ecstasy, union with God, trance -- is a manifestation of the ascent of this spiritual energy -- the raising of the kundalini.

 The initiation was never publicly revealed: only worthy disciples were taught it in direct transmission from their spiritual teachers, lest the powerful practices fall into the hands of immoral or power-seeking individuals. Thus for centuries this ancient science was passed only from the guru to chela, and never written down.

It is difficult for a modern man to understand the importance of guru-chela relationship and the significance of initiation. The modern man seeks happiness in materialistic comforts and worldly relationships. But the illumined sages say that only transcendental experience can lead to  real, eternal happiness and peace. That Happiness is billon fold more than the highest worldly happiness. The worldly happiness is merely in sense gratification, mixed ultimately with pain and misery. The more is sense gratification the more is craving for it, making the man restless, unsatisfied and unhappy. The worldly relations are based on self-interests and are ephemeral. The only true, eternal relationship is with God who is closer to man than his parents, siblings, spouse all put together. The Guru can connect theindividual with God. "To light a candle, you need a burning candle. Even so, an illumined soul alone can enlighten another soul." (Swami Sivananda). Contact with the guru guides ignites the inner flame of the  chela and the guidance of the guru keeps it burning brightly.