Watch out false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. – Matthew 7:15

A wolf in sheep's clothing is an idiom from Jesus’s Sermon on the Mount as narrated in the Gospel of Matthew.  It warns against individuals who play a deceptive role. The gospel regards such individuals (particularly false teachers) as dangerous.

Fables based on the idiom, dated no earlier than the 12th century CE, have been falsely credited to ancient Greek storyteller Aesop (620–564 BCE). The confusion arises from the similarity of themes in Aesop’s Fables concerning wolves that are mistakenly trusted, with the moral that human nature eventually shows through any disguise.

In the modern era, zoologists have applied the idiom to the use of aggressive mimicry by predators, whether the disguise is as the prey itself, or as a different but harmless species.

--Wikipedia

The phrase describes a person who appears gentle, friendly, or harmless on the outside, but is really dangerous, hostile, or out to do harm. The pleasant exterior is a disguise that hides a predator’s true nature. It is a warning to look past a charming or innocent appearance, because the most dangerous enemies are often the ones who do not look dangerous at all.

Source: https://proverbicals.com/a-wolf-in-sheeps-clothing

The image is ancient and comes to us from two great sources. It is the theme of one of Aesop’s fables, in which a wolf wraps himself in a fleece to slip in among the sheep unnoticed. It also appears famously in the Bible, where Jesus warns in the Gospel of Matthew (7:15): “Beware of false prophets, which come to you in sheep’s clothing, but inwardly they are ravening wolves.” Between the fable and the scripture, the phrase has been a byword for hidden danger for many centuries.

Source: https://proverbicals.com/a-wolf-in-sheeps-clothing

 

At its core, the metaphor symbolizes deception and hypocrisy. Just as a wolf takes on the disguise of a sheep to infiltrate the flock unnoticed, deceitful individuals cloak their true motives under a facade of righteousness, seeking to exploit and harm the unsuspecting.