It is only by practicing through a continual succession of agreeable and disagreeable situations that we acquire true strengths. To accept that pain is inherent and to live our lives from this understanding is to create the causes and conditions for happiness.
Happiness is sorrow; sorrow is happiness. There is happiness in difficulty; difficulty in happiness. Even though the ways we feel are different, they are not really different, in essence they are the same. This is the true understanding transmitted from Buddha to us.
When he bowed to all those buddhas, the buddhas he bowed to were beyond his own understanding. Again and again he did it.
So it is not a matter of whether it is possible to attain Buddhahood, or if it is possible to make a tile a jewel. But just to work, just to live in this world with this understanding is the most important point, and that is our practice. That is true zazen.
There is no need to have a deep understanding of Zen.