John 10:4

“When he has put forth all his own he goes before them, and the sheep follow him, for they know his voice.”

This is the test of which sheep are His. Once He has called them and put them forth from the fold they follow Him. And they can do so confidently, for as they go on the way, living their day by day lives, they know that He is constantly before them, watching over them and protecting them, leading the way. And they continually respond to Him for they know His voice. As He will say later in the chapter, ‘My sheep hear my voice, and I know them, and they follow Me --'.

Jesus probably had in mind here the prayer of Moses in Numbers 27:15. ‘Let the Lord, the God of the spirits of all flesh, appoint a man over the congregation, who may go out before them, and who may come in before them, and who may lead them out and who may bring them in, that the congregation of the LORD may not be like sheep which have no shepherd." And the Lord said to Moses, 'Take Joshua the son of Nun, a man in whom is the Spirit, and lay your hand on him”.' It is not without significance that the name ‘Joshua' is the Hebrew for ‘Jesus'. In the end Moses gives way to Jesus, of Whom Joshua was a type.