Galatians 5:9

‘A little leaven leavens the whole lump.'

They must therefore beware. For a small amount of yeast will spread through and affect the whole lump of pastry. So also will a small amount of false teaching affect a large number so that in the end all are affected. Or it can enter the individual mind and gradually possess it until that person is totally affected. Compare 1 Corinthians 5:6. Here he is stressing the danger that, when something like legalism begins to get a toehold, it is not long before it takes control of the whole. It spreads like yeast throughout the whole mixture. He might also be meaning that it only takes one man with false ideas to come in and then it can infect the group, and soon the whole group is led astray.