Beware of the leaven.

“… Look out; keep on your guard and beware of the leaven of the Pharisees and the leaven of Herod…” Mark 8:15 AMP

In other words, be on guard against hypocrites (wave makers, con-artists, cheats, fakes, phonies, frauds, pretenders) of this world!  As well, and probably most important, don’t let yourself become one of them!