When they went across the lake, the disciples forgot to take bread. (Matt. 16:5)
Jesus is not talking about yeast here. Jesus took the opportunity to draw an analogy using bread and yeast. So what is the analogy?
Jesus is comparing the teachings of these ecclesiastic institution's leaders to yeast. Why? Because yeast ferments, and through this fermentation, the bread rises. The yeast spreads throughout the flour during the process of rising, and insidiously saturates the bread as it begins to take shape, becoming part of the bread.
Organized ecclesiastical sectarian officials - men who gain their positions through appointment by men after political maneuvering - poison their followers with misinterpretations of scripture intended to attract followers and bolster their coffers.
This creation misinterpreted Jesus' teachings and his role substantially. The intent of the Synods was political. To retain their authority and control the population. Constantine was an emperor after all. This allowed the ex facto rule over Europe and the Middle East by the Roman Catholic church for many centuries.
Following the first Synod, the various scrolls regarded as scripture, which had been translated into at least 500 languages, were confiscated and translated into one language: Latin. Then this organization outlawed the reading of any other scripture other than their Latin version of the Cannonized Bible.
And because Latin was the language of the elite and those teachers of the early church, for over 1,000 years, practically no one could read the Biblical scriptures outside of the pope and the priests.
This absolute monopoly over the scriptures enabled the Roman Catholic Church to control its interpretation as well. As a result, there is one generally-accepted interpretation of Jesus' role and purpose even today - based largely upon the Nicene Creed written some 1,600 years ago for political expediency.
What this is is most certainly "yeast".
Still, because God is in ultimate control, there is enough of Jesus' teachings to provide clarity and evidence for his true role as the loving servant and representative of God. We see that the Supreme Being made sure this was not lost, and today with the findings of some of the Gnostic texts, we come to understand the brutal monopoly of the Roman Catholic yeast.
We also find the Supreme Being made sure the essential teachings of Jesus made it through these political organizations and their manipulations of scripture:
" 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment." (Matt. 22:37-38)
(For a translation of Jesus' statements from the Book of Matthew without institutional sectarian influence, see the Gospels of Jesus - translated from the original Greek texts.)