Few gods are as misunderstood today as the Greek god Dionysus, god of ecstasy, madness and wine. Today, he is often trivialized as a sort of frat-boy party god or sentimentalized as a happy-go-lucky hippy. But in his true, ancient form, he was much more, and the ecstasy that his presence evoked went far deeper than just giving one a good time, and was more dangerous than a romp in the mud.
In heavily simplified form, Dionysian ecstasy was an experience that, through its intensity, allowed the participant to temporarily go beyond their mortal body, and become one with Dionysus. Hence, why Dionysus is so heavily identified with wine and madness, which helped facilitate this experience. In this state of mind, it was common for the participants to demonstrate feats of incredible strength, often tearing apart living animals (which represented Dionysus) with their bare hands, and consuming its raw flesh- a ritual that has survived, in altered form, to this day in the Catholic communion. For this reason, many people, even in Classical times, feared Dionysus, and in Rome, his cult was entirely forbidden.
This is a very simplified explanation of a very complex religious figure and worship. A great introduction to Dionysus (and the study of religion in general) is Mircea Eliade's A History of Religious Ideas: Volume I, particularly the last section, "Dionysus: Bliss Recovered".
