Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber. But he that entereth in by the door is the shepherd of the sheep. To him the porter openeth; and the sheep hear his voice: and he calleth his own sheep by name, and leadeth them out. And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice. And a stranger will they not follow, but will flee from him: for they know not the voice of strangers. (John 10:1-5)
"The door" is the Holy Scriptures, the Bible. Blessed Theophylact, following the interpretation of St. John Chrysostom, explains this to us.
Jesus Christ Himself, the Son and Word of God, is also "the door", as He tells us a few verses later. It is only by Him that we can enter and be saved.
He is also the Good Shepherd, who enters by "the door" of the Holy Scriptures, since they testify to Him. Indeed, the whole Bible is about Jesus Christ. Everybody else speaks of himself, and twists the Scriptures to fit his ideas.
So how is it that we can be saved, and "go in and out, and find pasture"? Through Jesus Christ.
And how is it that we will recognize Him? Because He will "call us by name," and if we will "know His voice".
And how will we know His voice? Because we will have, throughout our lives, accustomed ourselves to hear and love His voice.
And how do we accustom ourselves to hear and love His voice? By reading the Holy Scriptures, by praying in the words of the saints, by attentively praying in the services of the Church, by frequently receiving the Holy Mysteries, and by striving to follow His directions about how to live, knowing that He will not lead us astray but will rather guide us into the good pasture.
So we are given two ways to recognize the Shepherd—one is by the way He enters into the sheepfold and the other is the sound of His voice. If the door is Holy Scripture then if what is being presented to us does not fit with what we read in the Bible then we know it to be false.
But many read the Bible and still are misled. That is why it is so important to be a sheep, a follower of the Shepherd, who recognizes his Master's voice…to read the Scriptures with humility. We become sheep of the Shepherd by either, through the grace of God, humbling ourselves, or through the difficult circumstances of life that a loving God has given to us in order to humble and save us
We are all sheep, whether we wish to be or not. We are either lost sheep, led astray, following after false teachings, ideas or passions. Or, through humility, we become His sheep, followers of the Truth who have come to recognize His voice.
"I am the Way, the Truth and the Life: no man cometh to the Father but by Me." John 14:6