What does Jesus mean by yeast?
The Greek word being translated to "yeast" - or "leaven" in some Biblical translations - is ζύμη (zymē) which can only mean "yeast" or "leaven" when used literally according to the lexicon.
When used metaphorically - as it was used by Jesus - the word means, "of inveterate mental and moral corruption, viewed in its tendency to infect others."
Thus, just as only a small amount of yeast will spread amongst an entire batch of dough - teachings based on corruption can infect a significant number of people.
This is confirmed by the word "hypocrisy" - translated from the Greek word ὑπόκρισις (hypokrisis) - which means "dissimulation" as well as "hypocrisy." The word "dissimulation" means "deceit, dishonesty, duplicity" and so on.
To this we can reference the many popular teachers over modern and not-so-modern times - including the Pharisees of the ecclesiastical Jewish institution that was corrupted - who have mislead many people over time. These are people whose teachings do not match their actions: Who may have been quoting Jesus' words (or in the case of the Pharisees, the various Prophets' words) but did not themselves follow Jesus' or these Prophets' teachings.
And just as the Pharisees were part of an organized institution during Jesus' time, many of these corrupted teachers have been part of the various sectarian ecclesiastical institutions that have claimed to represent Jesus.
Such is the irony but also the depth of Jesus' teachings - that he not only warned his current students of these corrupted teachers - but he warned all of us thousands of years later about the corruption that would take place among those institutions claiming to represent him.
The yeast of later sects
To this we can provide evidence of hundreds of years of persecution of innocent people by the Roman Catholic church and its various leaders who murdered hundreds of thousands - likely millions - of people over the centuries who didn't agree with their interpretations. Furthermore, this corruption continued even into modern times as children were molested by church priests while church leaders stood by and allowed these priests to evade arrest and continue their molestations.
Now they ask for our forgiveness, but we cannot provide this - this forgiveness can only be given by those whom they molested and in earlier times, burnt at the stake or committed other acts against. And it is only those who can provide forgiveness.
For the rest of us, we can only heed Jesus' teaching here:
"Be on your guard against ... hypocrisy."To this we can also reference Jesus' own criteria of who is acceptable to him. And this is the criteria we must have in accepting a teacher who claims to represent Jesus:
"Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?' Then I will tell them plainly, 'I never knew you. Away from me, you evildoers!'" (Matt. 7:21-23)So we find that "only the one who does the will of my Father who is in heaven" "will enter the kingdom of heaven."
What does doing "the will of my Father" mean?
It means doing what the Supreme Being wants. Just consider if we had a best friend and our friend said they wanted to go somewhere. What would we do? If we cared about them, we would take them to where they wanted to go. Even if we didn't want to go.
This is what doing the will of someone else means. It means thinking about what will please that other person, and then doing it.
This is what Jesus is speaking of: pleasing the Supreme Being. And this is precisely what drove Jesus' activities:
"By myself I can do nothing; I judge only as I hear, and my judgment is just, for I seek not to please myself but Him who sent me." (John 5:30)So Jesus is not only accepting that his empowerment comes from the Supreme Being; He is stating that his motivation for doing whatever he is doing is to please the Supreme Being - who sent Jesus.
This means two important things about Jesus:
1) He was the messenger of the Supreme Being
2) He was involved in an intimate loving relationship with the Supreme Being
This intimate loving relationship was not perceived by those who considered that Jesus threatened their ecclesiastical authority - the Pharisees. And it still is not perceived by many of the sectarian ecclesiastical institutions and their teachers even through today.
How do we know this? Because they claim that Jesus is the Supreme Being. They are confusing Jesus with whom Jesus is loving and serving - that Supreme Person whom Jesus asked all of his students to also love and serve:
" '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)