Before I do a post going in-depth into the view of the Vajrayana, I would like to set the view of setting the view. :) My understanding is, in all liberation theologies - which means actual spiritual paths that lead from a point A (confusion, selfishness) to a point B (wisdom, selflessness) - there are three methods available to make that leap. Some variation of these three is present in all the actual paths - religion at it's best, free from the body-count and ego-inducing influence of politics. These methods are true compassion, devotion, and wisdom via analysis.
In most paths, two of the three are present, and that ain't bad. :) For example, in the Gospel of Matthew (22:37-40), there is the explicit passage (quoting almost verbatum Deuteronomy 6:5 ) of there being only two commandments: love your neighbors as yourself and love Yahweh with your whole heart. 
(FYI, by this point Yahweh has been totally transformed away from the petty local war god (aka drahla, in Tibetan) he was originally conceived as to something much much bigger and more interesting) . Saul of Tarsus wrote that these two things were impossible, but simply believing in 'the Christ' was better. BULLSHIT. These two elements - true compassion and devotion beyond the normal mundane meanings of the phrase - cropped up all over the world as a way to get beyond the small 'self'. People actually accomplished these methods too. Therefore, thinking that no human being could possibly apply these can be proven to be false.
(FYI, by this point Yahweh has been totally transformed away from the petty local war god (aka drahla, in Tibetan) he was originally conceived as to something much much bigger and more interesting) . Saul of Tarsus wrote that these two things were impossible, but simply believing in 'the Christ' was better. BULLSHIT. These two elements - true compassion and devotion beyond the normal mundane meanings of the phrase - cropped up all over the world as a way to get beyond the small 'self'. People actually accomplished these methods too. Therefore, thinking that no human being could possibly apply these can be proven to be false.(I'm not exactly a fan of Saul/Paul - can't you tell? :))
It is totally possible. This is the way that various Hindu schools achieve their results too - the Shakti devotional yogas are a good example of the path of devotion. Back to that shortly.
Shakyamuni Buddha added a third method - wisdom via analysis, aka vipasyana. This was the revolutionary idea of taking a stilled mind (which is a by-product of the first two methods, btw) and applying it to an object of contemplation, pulling back the layers of it to find some truth about it. In particular, it is analysis of something (starting of course with that closest to oneself, one's actual "self") to find the true reality behind it.
Most of the Buddhist schools that developed had the true compassion and the vipasyana. As the dharma traveled, the methods for these were adapted as appropriate for the cultural context it entered - the various Chan/Zen schools, for example - but at heart it was still the same things. There was some devotional element to it - especially for the lay community who supported the monastic practitioners - but it wasn't very developed as a method of it's own.
That changed around the 3rd Century C.E/ A.D. Remember the Hindu devotional yogas I referred to before? One of the key methods of this was to take some 'deity' - some avatar (no, not from the #$@$@#$@ James Cameron movie :)), Krishna, Shiva, etc. - and keep in mind an idea that they are hanging out in the space above one's head. This form of practice is known as Guru yoga. This form of practice was originally invented as Karma-yoga - "action yoga" - by Hindus to allow 'spiritual' practice to occur off the meditation mat.
This allowed one to use the natural devotional/longing emotional response built into humans (which is most people is only expressed towards a sexual partner or would-be sexual partner) and redirecting it towards another object of 'faith'. It is basically committing a spiritual judo by taking that longing faith usually expressed towards a lover and aiming it instead at a higher idea present in iconographic form in front of one's self. Keeping this devotion up naturally cuts through ego ideas of 'me me me me' by cutting through one's always thinking about one's self. By doing this, it also means that one can keep up a spiritual practice while going about one's activities, since humans find it naturally easier to wrap their mind around some visual image and keep it there versus an abstract idea, which requires totally undistracted focus to 'get'. In short, this method became the basis for a 'meditation in action', which not only monastics/retreatants could do (such as the stereotypical Indian yogi sitting cross-legged all day somewhere by the Ganges), but also folks who had to work for a living.
Note: I am basing my views of the dawn of tantra on some works of Karen Armstrong, my notes of the late, great goofy Vajra nutball genius Dr. Robin Kornman, and my very imperfect notes of some history of Buddhism from Lochen Dharmashri, one of the greatest editor/collectors of Nyingma tantras in the last 500 or so years.
It came about in about the 3rd Century C.E. in what is now present-day Afghanistan and Northern India. Since Hinduism and Buddha-dharma have crisscrossed numerous times, these new techniques rapidly were adapted. This helped address a complaint within the lay community that they didn't have any practices they could do.
In addition, in monastic settings, there were lots of duties (cooking, building, administration, buying supplies, etc.) that meant there were people who had renounced worldly life that ended up not being able to actually practice towards liberation. Guru yoga, using the path of devotion, provided a potential solution to these problems.
So, to recap, there are three main ways of liberation/moksha/entering the Kingdom of Heaven. These are compassion, devotion, and wisdom via analysis/vipasyana. Most religious systems have two of them, which is enough to get one out of small mind. Mahayana Buddhism also calls true compassion "relative bodhichitta", and wisdom via analysis "absolute bodhichitta". Further, Vajrayana buddha-dharma has all three ways of liberation fully developed.
I don't mean to imply that because it has all three, the stuff I now do is superior to everything else. It just means there are more methods available to cut through emotional defilements. While having more is in some ways better, it also provides more potentials for pitfalls and steering off whatever path one is on and getting stuck in a ditch. :) This is the "Spiritual Materialism" that Trungpa Rinpoche famously spoke about, which will be addressed in more detail in a later post, if all goes well. Until then, Peace and stuff.
-JTR
