When Gautama realized ultimate reality (the Dharma) he emitted, simultaneously, spiritual light which was also compassionate. Like rays coming from the sun. Few people could sense it because it was so subtle and fine. The people who could sense it, were people who had developed spiritual roots over many rebirths. These where the people that he truly taught. In that time, if anyone sensed the Buddha's spiritual light/compassion which was luminous as well, they entered the stream of nirvana. The more these 'entered' followers let go of clinging to the the Five Aggregates (material shape, feeling, perception, volitional formations and sensory consciousness) the more of the spiritual light they became.
With the passing away of the physical body of the Buddha there were still some of his old followers who understood the Buddha's awakening at a very subtle level. They also understood that the spiritual light that he emitted, which was also compassion, could still teach the highest awakening. However, with the Buddha's physical passing the spiritual light of the Tathagata who, it is said, is the giver of light and vision, could still manifest itself. But then faith began to decline.
Of course, these days, all this stuff sounds like mumbo-jumbo. They can't be real occurrences. Many believe this kind of language is metaphorical which means certain things the Buddha said are not to be taken, literally. For if they were real occurrences, we should be in deep doo-doo, which is to say, we don't know what the Buddha is actually trying to teach. However, in my own example, the light of Mahayana I had experienced was at a very literal level and real beyond a shadow of a doubt. By it, I was able to understand Buddhism as opposed to just-guessing-Buddhism. What this all means for the average Buddhist, is there is a lot more to this religion than you can even imagine. But more importantly, you must have great faith in Buddhism. It won't let you down.
While all of this may sound exceedingly complicated and heady, it is because of a lack of spiritual light. I try to tell people, again and again, that what animates them is eternal and undying, the animated is not. It is finite, subject to birth and death. When one, through meditation, turns to the animative—meets it face to face—they see the Buddha which is spiritual light. This is what Bodhidharma said: Everyone wants to see this Mind, and those who move their hands and feet by its light are as many as the grains of the sand along the Ganges, but when you ask them, they can't explain it. . . .Why do they not see it?