“Be careful,” Jesus warned them. “Watch out for the yeast of the Pharisees and that of Herod.”
The ESV has, “Watch out; beware of…”
Yeast is a simple single celled fungi used in making all sorts of baked products but especially known for its use in bread making and alcohol production. Take note – it is a single cell, yet it has such great power to change the environment around it.
Jesus uses it as an illustration to warn his disciples about the basis for their lives – the yeast of the Pharisees and Herod might not be familiar terms for us today but the ideas behind them certainly are. Self-centred, self-reliance in sharp contrast to the Gospel focussed self-dying that Jesus advocated a few verses later,
“If anyone would come after me, he must deny himself and take up his cross and follow me. For whoever wants to save his life will lose it, but whoever loses his life for me and for the gospel will save it. What good is it for a man to gain the whole world, yet forfeit his soul?”
Although yeast does have a decreased viability over time it seems to be capable of lasting for a very long time. Make sure the old yeast in your life has been done away with, and the new yeast of Jesus has been brought in.