We all think of different things when we hear the word, but there would be few people indeed who had no reference whatsoever for initiation. For some of us, it brings to mind lighthearted pranks on the New Guy, or brutal fraternity hazing, or a series of membership tasks to overcome, or even just paying a ridiculous fee for a new gym membership.
For those of us in spiritual communities, however, Initiation is much more heavily weighted. Rituals of Initiation convey a sense of spiritual authority, access to deeper and more subtle levels of a mystery tradition, exposure to volatile secrets, or even actual rank within a spiritual body. Having experienced a ritual of Initiation, the candidate can rightfully expect to "shift gears" in the group, to be a more active participant, entrusted with power and a greater share of stewardship. Initiation conveys a great many things in spiritual traditions, all of which have one thing in common:
They are granted to the Candidate by a superior.
These benefits must be bestowed upon the Candidate by a member of higher standing, who is actually in a position to bestow them in truth and not just symbolically. This absolutely requires that the person performing the Initiation be superior to the Candidate in terms of knowledge, experience and authority. Otherwise there is nothing to bestow and growth is impossible. The sun grows the tree because the sun has more light and heat - it's a simple matter of physics. The tree does nothing for the sun, because it has nothing to offer and no way to get it there.
To fully understand what this means, we'll first need to explore what the spiritual process of Initiation does in terms of its effect on the Candidate. Rituals of Initiation exist to prompt a specific degree and direction of spiritual evolution in the new member, using symbolic words and actions to simulate the natural conditions which would cause the human soul to become greater. These natural conditions have existed since before humans, but often the very same tension which forces spiritual evolution on a person could just as easily kill them or drive them insane. Early man quickly learned to recreate these conditions in (relatively) safe, controlled environments in order to produce a shaman class which had all been exposed to the same mysteries, and whose souls had made the same quantum leap forward. This experience can only be recreated by those who know the proper conditions and how to guide the Candidate through them safely.
Side note: Sometimes the ritual doesn't "take", and the symbolism fails to prompt the necessary evolution in the Candidate, but this is rare. Properly composed and performed, a good initiation ritual will have the desired effect if the Candidate is ready and well-prepared.
In other traditions, Initiation involves the candidate being introduced to the principal spirits of that tradition, and being vouched-for by an experienced Elder who has been that way before and is well known to the spirits involved. This is a very important form of Initiation, because it establishes a safe and reliable working relationship with all the most powerful forces that tradition works with, and without this introduction the candidate is putting himself in grave danger.
Which brings us to the notion of "Self Initiation". Many neopagan traditions offer the idea of initiating oneself into their tradition, if a suitable Master is not available. The rise of occult publishing and later, the internet, has increased the spread of this idea. Personally, I cannot strongly enough convey my disapproval of this idea, naturally because of the fact that it's impossible but more importantly for the fact that it's dangerous.
Properly understanding the nature of Initiation and all the process entails, how on earth could it be possible to perform an act of Initiation on oneself? Rituals of Initiation require the Candidate be exposed to mysteries and symbolism which are to have a fresh effect, and also be made privy to certain teachings and new techniques, all of which is impossible to gift to oneself. The idea of properly initiating oneself is as absurd as literally hoisting oneself into the air by the belt. Not only is the Candidate's strength insufficient for the task, there is nowhere for him to stand to accomplish this. He literally has no foundation to perform this action.
Now, it's possible to self-dedicate in a tradition, to swear this oath or that oath to the deity of your choice. That's another thing entirely. A person can pray for a natural act of spiritual evolution, but given the stress involved this probably isn't wise. For all intents and purposes, there is no way to actually initiate oneself into any tradition. Period.
We've seen how it's impossible, and I also mentioned that it's dangerous. Initiation conveys a certain degree of spiritual authority, but this authority is only based on the practical application of having learned and applied new experiences. Without that experience, the authority is empty. I like to think of "Self Initiated" people as the mall-cops of the spiritual world - they're all bark and no bite, and when the chips are down they're simply not up to the task of performing with the same degree of excellence one could expect from a true Initiate.
Moral of the Story: Put in the time and effort to find a Master, and receive the mysteries in the proper manner. If you don't have it in you to do it right, do you think you have it in you to fix the mess you're going to make?