The best breads are made with yeast. They have that heavenly, just-made aroma that makes my mouth water. And that soft, cushiony texture that melts in your mouth.
Yeast breads take hours to make. Compared to "quick breads," that you mix and stick into the oven and they're done in minutes, yeast breads involve a process that generally takes hours: mix the starter in warm milk or warm water and sugar and let it grow in a warm place. When it's ready, mix in lots of flour and maybe some more sweetener. Cover the lump and put in a warm place for a couple hours, or until it doubles in size. Punch it down (literally) and roll it into a ball again, put back into a warm place and until it doubles again. Then form it into loaves or buns and bake until done, which for a loaf could be another hour. Hungry yet?
Yeast was well known in Bible times. (They had not yet discovered baking powder.) Bread baked without yeast was "unleavened," and resembled crackers more than loaves. (Matzoh is an example.)
Jesus warned his followers to "beware the leaven of the Pharisees." The Pharisees considered themselves to be the experts on the Law of the time. Jesus wasn't talking about their bread, but their teachings. I believe he was warning about the expanded interpretation the Pharisees were giving to the Law. They were distorting it to the extent that it was no good anymore. The law was no longer pure, as it was when it had been given to people by God. This can happen to yeast, as to laws. It was and still is a good warning.