Because they know his voice. Though he speaks here of ministers, yet, instead of wishing that they should be heard, he wishes that God should be heard speaking by them; for we must attend to the distinction which he has laid down, that he alone is a faithful pastor or shepherd [1] of the Church, who conducts and governs his sheep by the direction of Christ. We must attend to the reason why it is said that the sheep follow; it is, because they know how to distinguish shepherds from wolves by the voice This is the spirit of discernment, by which the elect discriminate between the truth of God and the false inventions of men. So then, in the sheep of Christ a knowledge of the truth goes before, and next follows an earnest desire to obey, so that they not only understand what is true, but receive it with warm affection. And not only does he commend the obedience of the faith, because the sheep assemble submissively at the voice of the shepherd, but also because they do not listen to the voice of strangers, and do not disperse when any one cries to them.

Footnotes

1 - The word pastor signifies shepherd, but, for the sake of the reader, who may not be aware of its etymology, it has been found necessary, in some cases, to employ both of the words, especially where the figure holds so prominent a place in the discussion. -- Ed.

He putteth forth - Or leads them out of the fold.
He goeth before them - He leads them, and guides them, and does not leave them. A shepherd spent his time with his flocks. He went before them to seek the best pastures and watering-places, and to defend them from danger. In this is beautifully represented the tender care of him who watches for souls as one that must give account.

He goeth before them - A sixth mark of a true pastor is, he gives them a good example: he not only preaches, but he lives, the truth of the Gospel; he enters into the depths of the salvation of God; and, having thus explored the path, he knows how to lead those who are entrusted to his care into the fullness of the blessings of the Gospel of peace. He who does not endeavor to realize in his own soul the truths which he preaches to others will soon be as salt without its savor; his preaching cannot be accompanied with that unction which alone can make it acceptable and profitable to those whose hearts are right with God. The minister who is in this state of salvation the sheep, genuine Christians, will follow, for they know his voice. It was the custom in the eastern countries for the shepherd to go at the head of his sheep, and they followed him from pasture to pasture. I have seen many hundreds of sheep thus following their shepherd on the extensive downs in the western parts of England.

And when he putteth forth his own sheep,.... The Ethiopic version reads, "when he leads them all out"; in order to bring them into proper pastures:
he goeth before them; in allusion to the eastern shepherds, who when they put out their flocks, did not, as ours do, drive them before them, and follow after them, at least not always, but went before them: so Christ, the great shepherd, goes before his flock, not only to provide for them, but by way of example to them; in many instances he is an ensample to the flock, as under shepherds, according to the measure of grace received, should be: he has left them an example in many respects, that they should tread in his steps:
and the sheep follow him; in the exercise of the graces of humility, love, patience, self-denial, and resignation of will to the will of God; and in the discharge of duty, walking, in some measure, as he walked.
For they know his voice; in the Gospel, which directs and encourages them to exercise grace in him, and to walk in the path of duty: this they know by the majesty and authority of it; and by the power with which it comes to their souls; and by its speaking of him, and leading to him; and by the evenness, harmony, and consistency of it. The Persic version renders the whole thus; "when he calls and leads out the sheep, they go before him, and their lambs after them, for they know his voice".

The sheep follow him: for they know his voice. Also literally true in the East as all travelers testify, but a stranger they will not follow, because his voice is strange. So true is it that when a traveler has changed dress with the shepherd for an experiment, they still have followed the disguised shepherd's voice and refused to listen to the voice of a stranger in the garb of their own shepherd.

He goeth before them - In all the ways of God, teaching them in every point, by example as well as by precept; and the sheep follow him - They tread in his steps: for they know his voice - Having the witness in themselves that his words are the wisdom and the power of God. Reader, art thou a shepherd of souls? Then answer to God. Is it thus with thee and thy flock?