'Be on Your Guard' in the Bible

Meanwhile the people had gathered in thousands, so that they trod upon one another, when Jesus, addressing himself to his disciples, began by saying to them: "Be on your guard against the leaven--that is, the hypocrisy--of the Pharisees.

Be on your guard! If your brother does wrong, reprove him; but if he repents, forgive him.

"Be on your guard against the Teachers of the Law, who delight to walk about in long robes, and like to be greeted in the streets with respect, and to have the best seats in the Synagogues, and places of honor at dinner.

Be on your guard lest your minds should ever be dulled by debauches or drunkenness or the anxieties of life, and lest 'That Day' should come suddenly upon you, like a snare.