a person’s actions show her true character
—My grandmother always taught me that what a person does is more important than what he says. Her favorite sayings were “Actions speak louder than words” and “By their fruits ye shall know them.”

In the Sermon on the Mount, Jesus said,

Watch out for false prophets, who come to you in sheep’s clothing but inwardly are voracious wolves. You will recognize them by their fruit. Grapes are not gathered from thorns or figs from thistles, are they? In the same way, every good tree bears good fruit, but the bad tree bears bad fruit. A good tree is not able to bear bad fruit, nor a bad tree to bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. So then, you will recognize them by their fruit [by their fruits ye shall know them]. (Matthew 7:15-20)

Both by their fruits ye shall know them and wolf in sheep’s clothing talk about recognizing what is really in a person’s heart.

Jesus spoke of sheep and wolves again when he sent his followers out ahead of him to the towns where he was going. Talking about the “harvest” of people entering God’s kingdom, he said,

The harvest is plentiful, but the workers are few. Therefore ask the Lord of the harvest to send out workers into his harvest. Go! I am sending you out like lambs surrounded by wolves [lambs among wolves]. (Luke 10:2,3)

Lamb among wolves now means “an innocent or naive person facing people who will try to take advantage of her or hurt her.”