If one can feel the purity on Yom Kippur, he can then begin the true inner avodah of Yom Kippur.
The first step of the avodah is to truly do teshuvah, earnestly, from the depths of your heart, and only Hashem can testify what level a person has reached in his heart. If one has indeed done true teshuvah, he will surely feel a palpable closeness with Hashem as a result. If he doesn’t feel the closeness, it means that he hasn’t done complete teshuvah.
If you do feel the closeness on Yom Kippur, you have reached the first step, and the next step is to feel the pleasantness of this closeness, and then desire to get closer to it, and closer, and closer.
After that point, comes the true test, the point of mesirus nefesh (self-sacrifice), of being willing to give up your life for this. During the rest of the year, you may not be consciously willing to die for this closeness with Hashem. But on Yom Kippur, you can reach a point where you are willing to give yourself up for this. You can now reach the point that Nadav and Avihu desired, of being willing to die, for more and more closeness with Hashem.
