A spiritual instinct!

(J.C. Ryle)

"He calls His own sheep by name and leads them out.
When He has brought out all His own, He goes on ahead of them — and His sheep follow Him because they know His voice. But they will never follow a stranger; in fact, they will run away from him because they do not recognize a stranger's voice." John 10:3-5

We have in these verses, a peculiar picture of true Christians. Our Lord describes them as sheep who hear the voice of a true Shepherd, and know His voice — and as "sheep who will never follow a stranger — they will run away from him because they do not recognize a stranger's voice."

The thing taught in these words is a very curious one, and may seem "foolishness" to the world. There is a spiritual instinct in most true believers, which generally enables them to distinguish between true and false teaching.
When they hear unsound religious instruction, there is something within them that says, "This is wrong!"
When they hear the real truth as it is in Jesus, there is something in their hearts which responds, "This is right!"

The careless man of the world may see no difference whatever between minister and minister, sermon and sermon. The poorest sheep of Christ, as a general rule, will "distinguish things that differ," though he may sometimes be unable to explain why.

Let us beware of despising this spiritual instinct. Whatever a sneering world may please to say, it is one of the peculiar marks of the indwelling of the Holy Spirit. As such, it is specially mentioned by John, when he says, "You have an anointing from the Holy One, and you know all things." (1 John 2:20) Let us rather pray for it daily, in order that we may be kept from the influence of false shepherds. To lose all power of distinguishing between bitter and sweet, is one of the worst symptoms of bodily disease. In the same way, to be unable to see any difference between . . .
  the gospel — and the law,
  truth — and error,
  Protestantism — and Popery,
  the doctrine of Christ — and the doctrine of man,
is a sure proof that we are yet dead in heart, and need conversion!