A.T. Robertson Commentary Matthew 16:12

"Then understood they that he bade them not beware of the leaven of bread, but of the teaching of the Pharisees and Sadducees." — Matthew 16:12 (ASV)

Then understood they (τοτε συνηκαν). First aorist active indicative of συνιημ, to grasp, to comprehend. They saw the point after this elaborate rebuke and explanation that by "leaven" Jesus meant "teaching."